What are Communicable Diseases

A communicable disease is an illness that is transmitted from a person, animal or inanimate source to another person either directly, with the assistance of a vector or by other means. Communicable diseases cover a wider range than the person-to-person transmission of infectious diseases; they include the parasitic diseases in which a vector is used, the zoonoses and all the transmissible diseases. It is this element of transmission that distinguishes these diseases from the non-communicable.

If diseases are communicable, then they present in an epidemic or endemic form, while if non-communicable as acute or chronic, as follows:

Communicable:

Non-communicable:

All these can occur at the same time and in the same place, but communicable diseases are more common in developing coun tries and non-communicable in the developed world.

Epidemic diseases devastate whole populations, as when measles ravaged Fiji, killing adults as well as children. Populations then have to start again from the survivors to recover their former strength. These are essentially young and growing populations. With endemic diseases, it is children who are particularly vulnerable, so there is a high birth rate to compensate. With so many young people in the population, chronic non-communicable diseases are uncommon, but as people live longer, such diseases become more frequent. Chronic non-communicable diseases, therefore, are a problem of older aged populations as seen in the Western world.

This division between the developed and developing world is purely artificial as far as diseases are concerned. When the plague, or black death as it was known, spread across Europe, it caused as much devastation as when communicable diseases were introduced to newly discovered nations by Western explorers. The population started again from the survivors as it has had to do in the developing countries. Just over a 100 years ago, measles was as serious a cause of childhood death in large European cities as it is in countries today without well-organized vaccination programmes. A tropical

© R. Webber 2005. Communicable Disease Epidemiology and Control, 2nd edition (Roger Webber)

environment is more favourable to many diseases than the cooler temperate regions, but even here, tropical diseases like malaria were once common in Europe. There is nothing new or different about these artificially divided parts of the world except for the resources that each is able to devote to the improvement of its population's health. Communicable diseases could be reduced to manageable proportions if sufficient resources, both in financial and educational terms, could be spent on them and much of the reason why certain diseases (as illustrated in Table 1.1) are more common than others is due to poverty.

The difference between communicable and non-communicable diseases was quite clear-cut. When it was an organism that was transmitted, the disease was communicable; otherwise the disease was classified as non-communicable. However, this strict boundary is becoming less well-defined as new suspect organisms are discovered or diseases, by their very nature, suggest a communicable origin. Various cancers are good examples; the link between hepatitis B virus (Section 14.13) and hepatocellular cancer is well established and is now being prevented by routine vaccination. Epstein-Barr virus (EBV) seems to be a pathogenic factor in Burkitt's lymphoma, but there is also a causal relationship with malaria; so controlling malaria (Section 15.6) in Africa and Papua New Guinea, where this tumour is found, could have a double benefit. The EBV might also have a causal effect in non-Hodgkin's lymphoma and nasopharyngeal cancer. Kaposi's sarcoma may well be transmitted by the sexual route as shown by the number of people with it who acquire human immunodeficiency virus (HIV) infection via sexual transmission, compared with those becoming infected from blood transmission, in which case the tumour occurs only rarely. The trematode worms Schistosoma haematobium (Chapter 11) and Opisthorcerchis sinensis (Section 9.5) are causative factors in bladder cancer and cholangiocarcinoma, respectively. As a result, their control as communicable diseases will also reduce cancer incidence. Helicobacter pylori, an organism that thrives in gastric secretions, is probably a causative factor in gastric cancer. The commonest cancer with a communicable cause is cancer of the cervix, which is due to infection with the human papilloma virus (Section 14.11). Prevention of this infection by vaccination, now under trial, offers the greatest hope of reducing this important cause of female mortality.

Equally intriguing is the possibility that atheroma has an infective cause or association. With arteriosclerosis being largely responsible for coronary heart disease (CHD) and a major killer in Western countries, the possibility of preventing an infective causal agent is attractive. Chlamydia pneumoniae has been found within atheroma lesions, but not normal arteries, while cytomegalovirus is able to infect the smooth muscle cells of arterial walls. The association of H. pylori and CHD now seems unlikely, but herpes virus 1 could induce an endothelial cell response. The cause will probably be found to be multi-factorial, but perhaps in the course of time, nearly all diseases will be shown to have a transmissible factor in their causation. Even road accidents, for which there does not seem to be a necessity to look for a predisposing cause as in a communicable disease, might be made more likely to occur due to infection with toxoplasmosis (Section 17.5).

The key to any communicable disease is to think of it in terms of agent, transmission, host and environment. These components are illustrated in Fig. 1.1, which will be used as a framework in the description of this section. There needs to be a causative agent, which requires a means of transmission from one host to another, but the outcome of infection will be influenced by the environment in which the disease is transmitted.

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