In the classification by transmission cycle, diseases fall into two main groups: the diseases where only humans are involved and those in which there is an animal reservoir or intermediate host. These are zoonoses, which are infections that are naturally transmitted between vertebrate animals and humans. They can be grouped according to the intimacy of the animal to the human being:

• domestic, animals that live in close proximity to man (e.g. pets and farm animals);

• synanthropic, animals that live in close association with man, but are not invited (e.g. rats);

• exoanthropic, animals that are not in close association with man (e.g. monkeys).

The importance of this type of classification is that it indicates the focality of the disease. As domestic animals are universally distributed, domestic zoonotic diseases are cosmopolitan, whereas at the other extreme, in an exanthropic zoonosis, such as scrub typhus or jungle yellow fever, it is quite possible for humans to live in the same locality, but separately from the disease area. Humans have no part in the disease cycle, but come into contact with it only when they accidentally enter the affected place (focus).

In zoonoses, the animal is all-important in control. In some diseases, such as the beef and pork tapeworms, good hygienic practice and inspection of the animal carcass may be all that is required to interrupt transmission. At the other extreme, a disease such as yellow fever can never be eradicated from the population even if every man, woman and child were immunized because the reservoir of disease remains in the monkey population. In a zoonosis, the animal reservoir is of prime importance and only by studying the ecology of the animal population can any rational attempt be made to control it.

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