infectious disease is caused by a living organism entering or infecting another living organism. They are sometimes called communicable diseases because they can be transmitted or communicated from one person to another
The theory that some diseases are caused by the invasion of the body by micro-organisms was put forward by the French chemist and microbiologist Louis Pasteur. Th English surgeon Joseph Lister and the German physician Robert Koch were also involved in the development of this theory. In the mid-19th century, Pasteur showed that micro-organisms in the air caused wine to go ‘sour’. In the 1860s, Lister showed that carbolic acid (phenol) acted as a disinfectant, and prevented disease in bones following surgery. In 1880, Robert Koch identified the micro-organisms that cause tuberculosis and cholera. Th theory that disease can be caused by micro-organisms is called the germ theory. Organisms that cause disease are called pathogens. A disease that is caused by a micro-organism infecting the body is an infectious disease.
After considerable work on micro-organisms as the cause of disease, Robert Koch put forward the following ideas (or ‘postulates’) that should always apply if a certain micro-organism causes a disease.
• The micro-organism must always be present when the disease is present, and should not be present if the disease is not present.
• The micro-organism can be isolated from an infected person and then grown in culture.
• Introducing such cultured micro-organisms into a healthy host should result in the disease developing.
• It should then be possible to isolate the micro-organism from this newly diseased host and grow it in culture. Th fist postulate establishes a link between the micro-organism and the disease. Th following three postulates prove that the metabolism of a specific living micro-organism, when transferred into a healthy host, causes the disease.
So, how do micro-organisms cause disease?
Bacteria : [Example – Pneumonia, cholera,pulmonary tuberculosis (TB)]
Bacteria release toxins as they multiply. These toxins affect cells in the region of the infection, and sometimes in other regions of the body as well. Bacterial diseases can be treated with antibiotics, as each bacterium is a true cell with its own metabolic systems, and is capable of cell division. Some bacteria invade and grow in the tissues of organs, causing physical damage.
Viruses : [Example – influenza (‘fl), AIDS, measles, common cold]
Viruses enter living cells and disrupt the metabolic systems of the cell. The genetic material of the virus becomes incorporated with that of the cell and instructs the cell to produce more viruses. Viruses cannot be treated with antibiotics as they are not true cells and are only active inside cells, which
antibiotics cannot enter.
Fungi : [Athlete’s foot, farmer’s lung]
When fungi grow in or on living organisms, their hyphae secrete enzymes. These digest substances in the tissues, and the substances produced are absorbed. Growth of hyphae also physically damages the tissue. Some fungi also secrete toxins. others can cause an allergic reaction (e.g. farmer’s lung).
Protozoa : [ Malaria, sleeping sickness]
Protozoa cause disease in many different ways.
How are disease-causing micro-organisms transmitted?
There must clearly be a source of infection. Th origin of micro-organisms that infect other people is called the reservoir of infection. This is the principal habitat from which an infectious agent may spread to cause disease. Reservoirs of infection include:
• human beings – the reservoir for many diseases, including the common cold, diphtheria and others
• other animals – for example: chickens, the reservoir for salmonella infections; mosquito, the reservoir for malaria
• soil – the reservoir for tetanus and many other pathogens
• water – the reservoir for Legionnaire’s disease, amoeba, cholera, etc.
• food – the reservoir for many diseases including typhoid
• contaminated objects – contact infections such as HIV/AIDS and trachoma
• air – the reservoir for pneumonia, tuberculosis, etc.
What other types of disease are there?
Before we answer that question, we should really define what we mean by ‘disease’. Th World Health Organization’s definition of health is ‘a state of complete physical, mental and social well-being’. But disease is less easy to define. It doesn’t mean just the absence of perfect health. If we are less fit than we might be, or if we are feeling depressed at the thought of too much schoolwork, that doesn’t necessarily mean we have a disease. A useful definition of disease might be ‘a condition with a specific cause in which part or all of a body is made to function in a non-normal and less efficient manner’. This definition could include diseases of all organisms – including plants. It could also include physical, mental and social aspects of disease in humans. Infectious disease is just one type of disease. Disease can be caused by a number of other factors.
• A person’s lifestyle and working conditions may result in human-induced diseases. Examples include many cancers, together with some forms of heart disease and fibrosis.
• Degenerative processes are often the result of ageing. Arthritis and atherosclerosis are examples of degenerative diseases.
• Our genes may lead to disease. Haemophilia and sickle-cell disease are examples of genetic diseases.
• Lack of nutrients in our diet may lead to deficiency diseases, including scurvy (caused by a lack of vitamin C) and kwashiorkor (caused by a lack of protein).
• Social activities can lead to disease. Social diseases, including alcoholism and drug addiction, may result in dependency on the drug, isolation, clinical depression and various levels of antisocial behavior
Eating too much food can result in obesity, which is regarded as a disease condition in itself, and can also lead to other diseases such as coronary heart disease.
In many cases, it is an oversimplification to place a disease in just one category. For example, atherosclerosis (laying down fatty substances in arteries) increases as we age, so it can be classified as a degenerative disease. But our diet influences this process. If we eat more saturated fat, more fatty substances are laid down in our arteries. There is also a genetic component – some people are at increased risk of this disease because of genes inherited from their parents. Stress and high blood pressure increase the rate at which atherosclerosis develops, and these can be the result of our lifestyle. Clearly, atherosclerosis does not fit neatly into any one category. It is best to consider such conditions as multi-factorial.
In some cases, there is an obvious ‘malfunction’ of an organ or system, without there being any obvious damage or physical sign of disease in the organ. Because of the malfunction, these diseases are called functional diseases, for example, heart disease. Several intestinal conditions fall into this category. In many forms of Irritable Bowel Syndrome (IBS), there is no sign of damage or disease in the large intestine, yet the large intestine does not function normally. Myalgic Encephalopathy (ME or Chronic Fatigue Syndrome) is another functional disease. In this condition, for no apparent reason, the sufferer is drained of all energy and the simplest task can be an extreme effort. However, some biologists believe that there must be some kind of abnormality in the organs involved in functional diseases. For example, there is some evidence to suggest that, in ME, many of the mitochondria (which release energy in respiration) are abnormal. If this is true, then it may be that functional diseases are really just like other forms of disease, but the precise cause is yet to be discovered.