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Infectious diseases

• How significant are they?

Infectious diseases caused by bacteria, viruses, fungi, protozoa and other parasites of the human body kill up to 17 million people every year. This means that of all the people who die in a twelve month period, up to a third of them die from infectious diseases. The majority of these diseases can be easily cured and indeed, for some of them, vaccination and prevention programs have been implemented in several countries. Nevertheless, despite efforts, infectious disease still remains a serious threat to the health of hundreds of millions of people worldwide. According to a World Health Organisation report in 1995 the ten biggest killers of the year were: (1) Acute infections of the lower respiratory tract including pneumonia with 4.4 million deaths, 4 million of which were children (2) Diarrhoeal diseases which caused 3.1 million deaths; the majority of which were children (3) Tuberculosis caused 3.1 million deaths (4) Malaria caused 2.1 million deaths; half of which were children (5) Hepatitis B infection caused 1.1 million (6) HIV\\AIDS and (7) Measles caused 1 million deaths each (8) Tetanus caused the death of 460,000 infants (9) Whooping cough caused the death of 355,000 children and (10) Intestinal worms killed approximately 135,000 people. 

In addition to the huge price being paid in human life, the socioeconomic development of many nations is crippled by the burden of these diseases. HIV\\AIDS is having devastating effects in Africa and Asia causing the deaths of large numbers of the workforce with resulting productivity loss. In the industrialised world, the costs of treating infectious diseases are staggering. In the United States alone, the estimated cost of treating intestinal infections is $30 billion annually. Illness due to the Influenza virus costs the Americans $5 billion in direct medical costs a year with and estimated loss of productivity of a further $12 billion.

• Emerging and re-emerging infectious disease

In the last 30 years there have been as many as 20 new infectious diseases identified, all of which have had a serious impact on human health. Some of the more significant examples of these emerging diseases and infectious agents are Acquired Immune Deficiency Syndrome (AIDS), Severe Acute Respiratory Syndrome (SARS), Hepatitis C, Escherichia coli 0157:H7, Ebola, Helicobacter pylori (a bacterium that causes stomach ulcers and is also associated with stomach cancer), Rotavirus (a major cause of infantile diarrhoea worldwide), Legionella pneumophila (the causes of Legionnaire’s disease) and Human T-lymphotropic virus which can cause leukaemia. Along with these new diseases a number of others like Tuberculosis, Gonorrhoea, Diphtheria, that were once considered to be under control due to the availability of antibiotics and vaccines, have begun to re-emerge.     

The obvious question to be asked is why these new infectious disease are appearing for the first time or re-emerging after years of apparent inactivity. There are several explanations for this. Re-emergence is the one that is probably the most easily explained. Due to the over use of antibiotics, resistance among a number of significant bacterial pathogens has developed, making it increasingly difficult to treat them. Examples include Tuberculosis and Gonorrhoea and many hospital acquired infections like Pseudomonas aeruginosa and Enterococcus spp. The re-emergence of Diphtheria seen amongst the member states of the ex-Soviet Union was due to the collapse of the health service and the relevant vaccination programs.

The emergence of new infectious diseases however is slightly more complex and is influenced by a number of different factors. One of the most important of these is related to population growth, together with increased and rapid urbanization, resulting in overcrowding and poor sanitation. These conditions create breeding grounds for infectious disease. Wars, civil unrest and natural calamities can result in similar situations of overcrowding and poor sanitation as refugees are grouped in camps and aid centres.   

Social change also plays a role in creating suitable environments for diseases to develop and spread. The clustering of children in day care centres and the elderly in nursing homes puts these groups under increased risk of infection. The rapid increase in international travel and the speed in which an individual can cross the globe means that infectious agents can be carried from one continent to the next in a matter of hours. Likewise, international trade, particularly food trade, can facilitate the rapid transport of infectious diseases at a worldwide level. The recent SARS and Avian influenza scares are prime examples of this.  

Ironically, as medical procedures such as organ and tissue transplantation, implantation of man made devices (heart valves and artificial joints) and chemotherapy improve the required immunosuppression in such procedures can, together with antibiotic resistance, result in the emergence of hospital acquired diseases that once would not have been considered a threat.

At the level of the infectious organism itself, genetic mutation is the most likely explanation for the emergence of new diseases. Viruses mutate rapidly explaining why SARS and Avian influenza, both diseases of animals other than man, have emerged as important diseases amongst humans. Likewise, Escherichia coli 0157:H7, a variant of the innocuous Escherichia coli, which many of us carry in our intestine, has emerged as a human pathogen due to genetic mutation.

Research and improved identification procedures can also result in the discovery of new infectious diseases as has occurred in the case of the bacteria, Campylobacter jejuni and Helicobacter pylori, discovered in the late seventies and early eighties and the viruses, Human metapneumonia virus, SARS, Hendra virus, Nipah virus, discovered in the last decade. Indeed, a number of diseases exists that still have unknown causes. These diseases are believed by some to be due to an, as yet unidentified, infectious agent. For example, diseases that have a possible virus origin include Alzheimer’s disease (a disorder that causes the gradual loss of brain cells), Multiple Sclerosis (an autoimmune disease that affects the central nervous system), Sjögren Syndrome (a chronic disorder of unknown cause characterized by a particular form of dry mouth and dry eyes), Kawasaki disease (a leading cause of acquired heart disease in children) and juvenile onset diabetes mellitus (a severe metabolic disorder caused by insulin deficiency). Some of the symptoms of Autism, for example, are thought to be caused by the toxins produced by Clostridial bacteria that have been found in larger quantities in the gut microflora of Autistic children compared to normal children. 

Infectious diseases will continue to emerge. To counteract them, rapid identification and response procedures need to be implemented. In addition, new drugs and drug targets (particularly for bacteria) need to be identified to replace many of the increasingly ineffective ones available. 

Biological weapons

Recent world events have highlighted the possibility of using microorganisms (viruses, bacteria fungi) or their products (toxins) as biological weapons. Important characteristics of biological weapons are low visibility, high potency, easy delivery, robustness and lethality of the disease caused. It is also important that they can be manufactured easily and quickly.

Successful attack by biological weapons could in theory result in thousands of dead or incapacitated and could cause significant disruptions to societies and economies. The World Heath Organization has estimated that 50 kilograms of the bacterium Francisella tularensis, which is the cause of Tulameria, released over a city of 5 million inhabitants would result in 19,000 deaths and 250,000 incapacitated individuals.

The Center for Disease Control and Prevention in the United States has created two categories of biological weapons based on four criteria which are (1) public health impact (2) delivery potential to large populations (3) public perception of the agent (public fear and potential for civil disruption) (4) special public health preparedness needs required to respond to the agent. Type A agents, which include the bacteria that cause Anthrax, Botulism, Plaque, and Tulameria and the viruses that cause Smallpox and Ebola, can be easily spread or transmitted from person to person, result in high mortalities and have the potential for major public health impact. They might cause public panic and social disruption and require special action for public health preparedness. Type B agents, however, are moderately easy to spread and result in moderate morbidity and low mortality. These include Escherichia coli 0157:H7, the Ricin toxin and the bacteria that cause Brucellosis, Glanders, Melioidosis, Psittacosis, Q-fever, Salmonellosis,  Shigellosis  and Typhus fever.

In 1942 the small Scottish island of Gruinard was used by the British to test Biological weapons. They were interested in the effects of anthrax and so 80 sheep were brought to the uninhabited island and a bomb containing anthrax spores was exploded. The sheep began to die within 3 days and despite clean-up attempts the island was kept in quarantine for over 40 years. In 1986 a company was paid £500,000 to decontaminate the island which they did by soaking the soil in diluted formaldehyde. In 1990 Gruinard was official declared safe, 48 years after the initial contamination.

Protection against infection

• The immune system

The human body is continuously under attack from foreign agents. These agents, which are referred to as antigens, may be living entities, such as bacteria, viruses, fungi, protozoa or inanimate material such as dust particles, pollen, metals and food. The immune system is the mechanism by which the body protects itself against this onslaught of foreign material. It can be divided into innate and adaptive immunity.

Innate immunity includes the structural barriers created by our body to keep out invading material. There are substances in the tears from our eyes and in the saliva from our mouths that are harmful to bacteria. The skin, which can be as thick as 5mm in some places, covers the human body and provides an extremely efficient primary barrier against an invading microorganism. Oil (or sebum) is secreted from pores on the skin that keeps it slightly acidic, discouraging many bacteria from growing. Bacteria that enter the body through the skin can only do it if the skin is already broken by a wound or a scratch or if the skin is purposely broken by a third party like an insect or animal bite.

Bacteria and other particles in the air can enter the body through the respiratory tract. However, mechanisms exist which remove the bacteria before they can set up an infection. The hairs in the nose are capable of removing large particles containing bacteria as they pass through the nasal passage. Smaller particles trigger coughing or sneezing reflexes which expel them from the body. If bacteria manage to reach the lungs, a mechanism called the mucociliary escalator exists that removes them. This escalator is made of tiny hair like cells called cilia that beat in an upward movement so that bacteria and other unwanted material are moved up and out of the lungs. 

Fever is a mechanism that the body uses when a significant infection is underway. A simple localised wound infection, for example, will not result in fever. The mechanism is triggered by substances produced by blood cells when they come in contact with foreign bodies like bacteria. These substances then regulate the temperature control settings of the body resulting in an increase in temperature. This increase in temperature is an attempt by the body to upset the invading bacteria which are more comfortable at lower body temperatures. 

Our blood also contains anti-bacterial substances. Indeed, our blood contains a huge variety of cells that are referred to as white blood cells to differentiate them from the normal red blood cell which carry oxygen throughout the body. These white blood cells arrive at the site of invasion of a foreign body and either phagocytose (eat up) the invader or release toxic substances that kill the invader. They are also involved in the repairing of damaged sites e.g. the formation of a scab on a cut.  

The adaptive immune response is more complex again. It is a system whereby antigens are recognised as foreign based on their chemical fingerprint. Once activated, the adaptive immune response produces antibodies and cells that directly fight off the infecting agent. The specific chemical fingerprint is also remembered by the adaptive immune response so that it can react quicker the next time the antigen enters the body. Vaccines activate the adaptive immune system as a preventive measure against future possible infection. The adaptive immune system, however, is not perfect. Several microorganisms are capable of changing their chemical fingerprints to fool the immune system so that recurring infections can occur. This phenomenon is called antigenic variation and is seen in the bacteria Neisseria gonorrhoea and Borrelia recurrentis and in the viruses that cause influenza and the common cold. 

Susceptibility

• The immunocompromised 

Having discussed the complexity of the immune system above, it becomes clear how vulnerable people without a healthy immune system are to infectious disease. When we talk about immunocompromised patients the first group that we think of are those suffering from AIDS. In these patients, the HIV virus has infected and killed many of the cells involved in fighting disease and so individuals with AIDS become extremely susceptible to infectious disease. However, AIDS sufferers are not alone in having a reduced immune system. There are several other important categories of patients whose immune system does not function correctly. These include transplant patients, patients undergoing chemotherapy for cancer, chronic alcoholics, diabetics, the newborn, the elderly and the malnourished. In the first two cases, the immune system is suppressed by the drugs used to prevent rejection of the transplanted organ and as a side effect of the powerful anti-cancer drugs used in chemotherapy. In chronic alcoholics, a decrease in the functioning and number of the cells involved in the immune response occurs due to alcohol consumption. Indeed in the US, infectious disease is the number one killer of alcoholics. In diabetics, the immune system is affected by a process called glycation which can weaken protein structures in the body due to toxic sugar levels in the blood. This, in turn, results in a weakened immune system. In the newborn the immune system is underdeveloped and naïve while in the elderly the system beings to deteriorate with age. Finally, in the malnourished the low level of nutrients has an obvious effect on the proper functioning of the immune system. 

Introduction to bacteriology 

• What is a bacterium?

Bacteria are single-celled microorganisms without a nucleus that although they appear to be relatively simple forms of life are in fact sophisticated and highly adaptable. Structurally they are not related in any way to the other microorganisms that can cause disease like viruses, fungi or protozoa. Viruses are bundles of genetic material surrounded by a protein coat that can only replicate inside cells and are much smaller than bacteria. There are viruses called bacteriophages that actually infect bacteria. Fungi have a nucleus and, being more similar to plant cells, are more complex than bacteria or viruses. Protozoa are the largest and most complex of the microorganisms that cause disease. Indeed, bacteria are actually a source of food for protozoa.

Bacteria are everywhere. They are in the food that we eat, the air that we breathe and the water that we drink. They appeared on earth over 3.7 billion years ago and have colonised virtually every environmental niche since. To date approximately 4,000 species of bacteria are known to man but it is thought that there are many millions that remain to be identified. Bacteria are found in the coldest, hottest, driest, wettest, deepest and highest places on earth. They can survive in the most acidic, alkaline and salty of environments. They can eat virtually anything from dead bodies to car tyres. They have been found growing in toxic waste dumps and sites contaminated by radiation. They can help certain plants grow by accumulating nitrogen from the atmosphere. They are involved in the production of various foods and beverages like yoghurt, cheeses and wine. Without bacteria there would be no degradation. We, as humans, cannot live without them. Our bodies are covered in billions of bacteria that provide us with nutrients that we cannot supply ourselves and they help us digest our food. 

Bacteria have extremely fast replication or generation times relative to other living organisms and these times can vary from minutes to days depending on the bacterium. As they replicate, it is possible for each new generation to be slightly different to the next at the genetic level due to the introduction of spontaneous mutations. These mutations can have either a negative or positive affect on the bacterium depending on what the mutation has changed from the previous generation. A mutation that produces a positive effect may help the bacterium survive or adapt to a new environmental situation. If the mutation is negative the bacteria will not flourish and may die. Accumulation of these genetic mutations over generations can lead to a bacterium that can adapt to a particular niche environment and this explains why they are found in such a vast number of different environments.

Most bacteria are so small (between 0.5 and 5.0 micrometers) that we cannot see them without the help of a microscope. A micrometer is a millionth of a meter. The first descriptions of bacteria were by Anton van Leeuwenhoek in 1677 using a single lens microscope of his own design. Recently, however, a bacterium was discovered in the intestines of a fish off the coast of Australia that can be seen with the naked eye. Epulopiscium fischelsoni is about the size of the full stop at the end of this sentence. 

• Bacterial names

One of the first obstacles when entering the world of bacteriology is the nomenclature (naming) procedures used. All of the names are in Latin and therefore provide pronunciation problems for the majority of us. Similar to the first and last names that we ourselves use, bacteria are called by their genus name (equivalent to the family name) followed by their species name (equivalent to the first name).  For example, the Bacillus family is made up of several members e.g. Bacillus anthracis, Bacillus cereus, Bacillus licheniformis, Bacillus pumilus, Bacillus subtilis etc and although they are from the same family, each member has different characteristics. A human equivalent might be the Jackson Family whose members have obvious similarities but can still be easily distinguished from each other. Michael Jackson is as different from Germaine Jackson as Bacillus anthracis is from Bacillus cereus. The members of the Osmond family would again share similarities but have very little in common with the Jackson’s much the same as Bacillus subtilis has little in common with Yersinia pestis.

In microbiology, when we refer to the genus of bacteria without naming all its members the term spp. is used. Therefore, all of the Bacilli which include Bacillus anthracis, Bacillus subtilis, Bacillus cereus, Bacillus pumilus and many others are referred to as Bacillus spp.

In certain cases, the name itself either identifies the discoverer of the bacterium e.g. Brucella spp. (discovered by Bruce), Salmonella spp. (discovered by Salmond) Yersinia spp. (discovered by Yersin), gives an indication as to the what the bacterium does, e.g. Legionella pneumophila (pneumophila literally means “lung lover”), Neisseria meningitidis (causes meningitis), where it is found Helicobacter pylori (the pylorus is a region of the stomach) refers to the shape or size of the bacterium e.g. Mycobacterium spp. (Myco means minute), Helicobacter spp. (helical or spiral shaped). 

• Normal flora

From shortly after we are born to when we die we are colonised by a large variety of different bacteria which are referred to as normal flora. It is estimated that the average healthy human is colonised by 100,000,000,000,000 (10¹⁴) bacteria at any given time. Specific bacteria colonise different sites of the body, which include the skin, the oral and respiratory tract, the gastrointestinal tract, the urinary tract, the genitalia and the eyes. These bacteria can be beneficial, harmful or neither to their human host. They may provide nutrients to us that we cannot produce ourselves (e.g. Vitamin K) and they can occupy sites in the body that could otherwise be occupied by more harmful bacteria.  

The normal flora in humans usually develops over time in an orderly sequence after birth and results in stable populations of bacteria that make up the normal adult flora. For example, an infant is colonised by bacteria that it comes into contact with as it moves through the birth canal. A particular flora is set up and flourishes in the gastrointestinal tract if the infant is breast-fed. This bacterial population is reduced and displaced somewhat when the baby begins bottle-feeding. As the child grows and it comes into contact with different foods and environments the normal flora will change accordingly.

The main factor determining the composition of the normal flora in a body region is the nature of the local environment, which is determined by pH (a measure of acidity or alkalinity), temperature, oxygen, water, and nutrient levels. The different skin regions of the body have been compared to geographic regions on Earth: the desert of the forearm, the cool woods of the scalp and the tropical forest of the armpit.

Despite the benefits of normal flora they can also cause infection by taking advantages of changes in the body. Dental caries and periodontal disease, which affect about 80% of the population in the Western world, are caused by some of the 350 different bacteria that make up the normal flora of the mouth. In addition, many bacteria that cause infections of the brain, face, and lung originate from the oral normal flora. 

• Why do bacteria cause disease?

Like every living thing on earth, it is of vital importance for bacteria to reproduce and pass on their genetic information to the next generation. Many bacteria do this in harmony with their environment by existing in a free-living or parasitic form. Of the parasites, only a very small portion of them actually cause disease and this characteristic can actually be seen as a failure of the bacteria to adapt to its environment. In fact, a well-adapted parasite ideally thrives in or on its host without causing significant damage. Killing or damaging your host is not a good method of safe-guarding the species, particularly if you don’t have a suitable means of being transmitted to a new host.

One advantage for many bacteria is the speed in which they multiply. Many medically important bacteria like Escherichia coli, Salmonella spp. and Bacillus spp. take just 20 to 30 minutes to replicate. This means that under optimal growth conditions (i.e. with all the required nutrients and the correct growth temperature) in 20 minutes 1 bacterium has become 2, 4 in 40 minutes, 8 in 60 minutes and 4,722,366,482,869,645,213,696 in 24 hours. This does not happen in reality because of environmental and nutritional restraints and also because, in a bacterial population, an equilibrium is reached between the bacteria that are dying and those that are replicating. Not all bacteria are as quick at replicating. Mycobacterium tuberculosis, the cause of Tuberculosis, can take up to 10 hours to replicate. 

Diseases caused by bacteria

You will notice from reading the descriptions of the bacteria in this book that a lot of them cause the same type of disease in humans. The five most common diseases are diarrhoea, endocarditis, meningitis, pneumonia and septicaemia all of which can be fatal unless treated. 

• Diarrhoea is described as the passing of significant quantities (up to 300g in 24 hours) of loose stools and is usually due to a bacterial or viral infection. It is estimated that 7 children die every minute of diarrhoea worldwide. Most cases of bacterial diarrhoea are due to the consumption of contaminated food or water. Symptoms can include fever, abdominal pain, vomiting and dehydration. The bacteria attach to and irritate the lining of the small or large intestine, which reacts by releasing a large amount of water with the faeces at irregular periods. Blood can be sometimes found in the stools due to the action of bacterial toxins, which damage cells. In many cases, diarrhoea is self-limiting, so no antibiotic treatment is required. In more severe cases, when there is fever or blood in the stools, medication and hospitalisation may be necessary.

• Endocarditisis an inflammation of the inside lining of the heart chambers and heart valves. It can be caused by both bacteria and fungi and is generally seen in patients that already have underlying heart disease. Due to slight deformities in the heart, bacteria that have entered the bloodstream can attach to the lining of the valves and multiply. In a normal healthy individual, the valves have a smooth lining making it very difficult for bacteria to attach. Common symptoms of endocarditis are fever, chills, night sweats, muscle pain, shortness of breath, blood in the urine and paleness. Hospitalisation and long term antibiotic therapy is usually required which, in some cases, can last up to 6 weeks.

• Meningitisis an infection of the meninges, which is the fluid surrounding the brain and spinal chord, by a bacterium or a virus. Symptoms include a high fever, stiff neck, acute headache, vomiting, discomfort when looking into bright lights, confusion and, as the infection progresses, seizures. Long-term complications of meningitis may include brain damage, learning disabilities and hearing loss. It usually occurs in children and young adults and can be fatal if not treated promptly. Antibiotics are used to treat the disease and hospitalisation is normally required.

• Pneumonia is an inflammation of the lungs due to an infection by a microorganism which is generally either a virus or a bacterium. Bacterial pneumonia normally begins as an infection of the nose or throat from which the bacteria spread into the lungs but can also be caused by bacteria that are carried from another site of infection in the body through the blood to the lungs. Pneumonia is usually associated with a patient whose immune system has been weakened either by ill health or age. Symptoms include fever, chills, cough, rapid breathing with wheezing, chest pain, vomiting, decreased appetite, and, in extreme cases, bluish colouring of the lips and nails. It is estimated that 1.2 million people are hospitalised each year for pneumonia, which is the sixth leading cause of death in the US.

• Septicaemia is the presence of bacteria in the blood. The bacteria enter the blood from other sites of infection, like the lungs or the urinary tract. It progresses extremely rapidly and is highly dangerous. If not treated in time it can lead to septic shock and death. Septic shock is characterised by a decrease in body temperature and blood pressure, confusion and haemorrhaging under the skin. Other symptoms include fever, chills, rapid breathing and increased heart rate. Hospitalisation is essential and intensive care is usually required. Antibiotics are administered intravenously. Mortality is extremely high at 50%.   

Treatment and protection against bacteria 

There are a number of ways that infectious disease caused by bacteria and viruses can be treated and prevented. It is important to underline, however, that some treatments are only suitable for one type of organisms. For example, antibiotics only have an effect on bacteria and are completely useless against viral infections. Indeed, the over consumption of antibiotics for the treatment of infections caused by viruses has played an important role in the emergence of bacterial resistance to antibiotics. Likewise, antivirals are, as the name suggests, used to treat viral infections and nothing else. Vaccination, on the other hand, is used to prevent both bacterial and viral infections.

• Antibiotics

It was a Scottish scientist, Alexander Fleming, who discovered the first antibiotic in 1928. Penicillin is a substance produced by a mould called Penicillium that is commonly found on bread. Fleming showed that bacteria could not grow on a Petri dish containing the Penicillium and that it was actually a substance released by the mould that prevented the bacteria from growing. 

It took until 1943 for scientists to work out a way to produce Penicillin on a large scale. Over the following decades pharmaceutical companies began to discover new antibiotics like chloramphenicol, tetracycline and vancomycin or they produced more efficient chemical cousins of the original penicillin like ampicillin, methicillin and oxacillin (if the drug’s name ends in “illin” it is a derivative of penicillin). By the 1980’s, complacency had set in as it was felt that man had finally conquered bacterial infection. Just as many companies stopped working on the discovery of new antibiotics, the development of antibiotic resistance among bacteria was beginning to be recognised as a serious future threat.

Bacteria have evolved to counteract the action of antibiotics so quickly that by the 1990’s the first reports appeared of bacteria that were resistant to all known antibiotics. One of the main reasons for this is generally thought to be the over-prescription and over-use of antibiotics. A natural selection process occurs whereby spontaneous mutations change the site of action of the antibiotic in the bacterium so that the antibiotic is no longer effective. Obviously, bacteria with these changes survive to reproduce and the changes are maintained in the following generation. An example would be the resistance to Penicillin. Penicillin kills bacteria by binding and destroying a key part of the bacterial cell wall. The wall falls apart and the bacteria die. In resistant bacteria, the bacterial cell wall is slightly changed so that the antibiotic can no longer bind.

Antibiotic resistance has resulted in the re-emergence of bacteria that were believed to be, if not conquered, at least under control. Tuberculosis, caused by Mycobacterium tuberculosis has once again become a serious health problem due to antibiotic resistance. Likewise, hospital acquired bacterial infections due to antibiotic resistance in Pseudomonas aeruginosa, Staphylococcus aureus (MRSA), Staphylococcus epidermidis and Enterococcus spp. are steadily increasing.

• Vaccines

The English doctor, Edward Jenner was the first to use vaccination in 1796. After observing that a milkmaid was unable to acquire small pox, a deadly viral disease characterised by pus filled blisters, because she had previously contracted cowpox, a similar but less serious disease of cows, Jenner began to prepare for the first ever experimental vaccination. (Vaccination is derived from the Latin word “vacca” for cow). In a highly unethical experiment, he infected a young boy (thought by many to have been his own son) with cowpox, allowed him to recover and then infected him with the small pox virus by injecting pus from a small pox blister under the boy’s skin. As Jenner predicted, the boy did not develop small pox.   

Although Jenner did not know it at the time his vaccination experiment worked due to the fact that when a foreign body enters a healthy human body, an immune system exists to fight off the invader. As discussed previously, the adaptive immune system is involved in producing antibodies against antigens and fighting the infection. Even when the infection has been successfully removed a number of these antibodies, which are produced by cells called memory cells, will remain circulating in the blood of the individual. Usually, if the body comes across the same foreign body again at a later stage which can be months or years later, the memory cells will recognise and destroy it rapidly before infection can get a hold. This is why there are some diseases like measles and mumps that we can only get once in a lifetime. In the case of Jenner’s experiments, the cow pox and small pox viruses were similar enough to each other to be recognised as the same antigen by the immune system. The boy infected with cow pox had developed antibodies that were also effective against the subsequent small pox infection. 

Bacterial vaccines can be of various forms. They can contain whole dead bacteria, parts of bacteria that are not infectious or inactivated toxins produced by the bacterium. 

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