No biological system is perfect and communicable diseases in particular are not readily classified; however, any grouping makes it easier to understand and remember, so the objective of this short chapter is to look at the different ways this can be done.
A disease is a morbid condition of the body (e.g. measles or plague). As the cause of diseases were discovered, they became identified by the causative organism, such as trypanosomiasis or pneumococcal meningitis, but confusion arose because there are two forms of African trypanosomiasis and one of American, while the pneumococ-cus is an important cause of pneumonia as well as meningitis. This confusion continues. Instead of settling on one system or another, I have tried to list all the communicable diseases by either the disease state or the organism by which they are best identified. For example, in the case of gastroenteritis, one of the commonest causes of diarrhoea in developing countries, it is preferable to separately list the various organisms that can cause it. Where these are particularly distinct, such as rotavirus infection, then they are put into the list. On the other hand, the streptococcus is responsible for such an array of diseases that just to put down streptococcal infection would fail to reveal important diseases such as rheumatic fever or otitis media. With these provisos, all the communicable diseases are listed in Chapter 19, at the end of the book, rather than here, so quick reference can be made to them.
There are 340 diseases listed in Chapter 19, of which the commonest causative organism is a virus, being responsible for 185 diseases, with arboviruses causing 118 infections. Bacteria and chlamydia account for 66 and the larger parasites for 54 infections, of which the nematodes cause 21 diseases, protozoa 17 and helminths 16. The commonest method of transmission is, therefore, the vector, with the mosquito being incriminated in a staggering 76 infections, ticks in 31, and other or unknown biting insects in 42 of the infections. In methods of control, with vectors being so frequent, vector control is the commonest, which proved useful in 134 of the disease conditions, but simple methods, such as using repellents and sleeping under mosquito nets, are all that are required most of the time. Next comes personal hygiene, invaluable for preventing 90 infections - just washing your hands, not spitting and making an effort to be clean can be remarkably effective. Allied to personal cleanliness is food hygiene, ensuring that food is prepared properly, adequately cooked and stored under safe conditions, accounting for 57 of the preventive methods. Allied to the proper cooking of food is the control of animals either in their farming, slaughter, and the control of domestic pets, this group being responsible for 41 disease preventive actions. Chemotherapy as a method of control is valuable in 40 diseases and vaccination in 35, but several of these are major disease problems, such as tuberculosis, measles and the STIs. The provision of a good water supply will reduce 39 conditions and sanitation another 23, while the control of rats is important in 17 conditions. The social and educational methods appropriate to controlling STIs will be valuable in 20 conditions, while the screening of blood donors will avoid 14 diseases and the proper sterilization of needles, instruments and giving sets could prevent nine diseases.
No attempt is made in the next few chapters to cover all 340 of the diseases listed, but to select only those of worldwide importance, which are major problems in certain parts of the world, or to illustrate a particular disease pattern. Readers might find it useful to refer to the list in Chapter 19 first, before turning to the fuller description in the following pages.
Diseases are normally classified by the causative organism, which has much to recommend it for the clinician and the pathologist, but different organisms can cause similar diseases, such as Escherichia coli, a bacteria and Giardia intestinalis, a protozoa, both producing diarrhoea in the individual. Control methods are similar; as a result, an epidemiologist will find it preferable to include them in the same group. On the other hand, the closely linked group of viruses that cause hepatitis are very different in their means of transmission -hepatitis A is transmitted by the faecal-oral route and hepatitis B by blood and other body fluids, so it is preferable to separate these two diseases into different categories. Transmission is the key to the epidemiology of communicable diseases. Once the means of transmission is known, it leads to the best method of control, so it is preferable to use this as the method of classification. Based on these criteria, all communicable diseases can be classified into 11 groups as follows:
Water washed diseases 7
Food-borne diseases 9
Diseases of soil contact 10
Diseases of water contact 11
Skin infections 12
Respiratory diseases and other airborne 13
Diseases transmitted via body fluids 14
Insect-borne diseases 15
Ectoparasite zoonoses 16
Water-washed diseases could be called person-to-person diseases, but many diseases including skin infections and respiratory diseases are also transmitted from one person to another, so a preferable description is to include the main method of control, which is washing. They could also be called diseases of poor hygiene, but since hygiene is involved in the control of very many diseases, to call them this would make this category far too large. Faecal-oral is a very large group and could quite easily incorporate many of the diseases in the chapter on food-borne diseases, but it is easier to consider control methods if a separate chapter is made. There are important diseases that are acquired by contact with either soil or water, which means that methods of control are very specific. Skin infections are obvious in their presentation and most of them are transmitted directly by skin contact but also use other modes of transmission, so it is more convenient to classify them by their most common method of presentation. Instead of putting leprosy in a separate chapter as in the previous edition, because its means of transmission has not been fully worked out, it is included in the chapter on skin infections. The respiratory infections are transmitted by the airborne route, which is also a method of transmission of several other infections that present in different ways. The chapter on diseases transmitted via body fluids is an attempt to bring together common themes in the transmission and control of diseases transmitted via blood, seminal fluid, cervical secretions, saliva and other less common methods of transmission. It includes the STIs, which would warrant a chapter of their own, but other diseases that share many common features, such as hepatitis B and non-venereal syphilis, are better included with them. Insect-borne diseases not only include a large number of health problems, but also some of the most important diseases in the world such as malaria. It is already the largest chapter and could be even bigger, but the combination of ectoparasite transmission (by fleas, lice, etc.) and zoonosis is a very specific one, so a separate chapter has been included for this category. The rest of the zoonoses, where a vector is not included, form the last chapter in the classification.
No classification system is perfect and not every disease fits neatly into the 11 categories. For many diseases, there is more than one means of transmission and these can also be important in developing control methods. However, the categories are sufficiently broad to encompass minor differences. Bringing them together into such a system demonstrates similarities and associations, making it easier to understand the complexities of the many communicable diseases.
The fewer groups there are, the easier it is to remember all of them, sparing the onerous task of learning about each disease in detail. However, if each group is too broad, much of the essential information is also lost, thereby defeating its purpose. For example, Vietnam had classified all its communicable diseases into just four groups, but this was found to lack the precision to work out the best control strategy for each group, so this was replaced by an abbreviated classification, as follows:
1. Person-to-person (skin and eye diseases).
2. Faecal-oral transmission.
4. Airborne (respiratory infections).
5. Diseases transmitted via body fluids (includes STIs).
6. Vector-borne diseases.
This simplification was due to the absence of such diseases as schistosomiasis and Guinea worm, which are the only members of the 'diseases of water contact' in the classification above, and several others, which allowed amalgamations. Any country might similarly like to draw up its own classification system based on the important diseases found there.
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