The zoonoses, which are defined as infections of man naturally acquired from other vertebrates, are treated as a problem in ecology. This entails studying the interrelation between man, animals, a causative organism, the environment, and sometimes arthropods. Such an approach is holistic or synecological—wholes being regarded as more important than parts. Holism is a dynamic not a static conception. The evolution of the zoonoses is discussed, particularly in relation to plague, relapsing fever, and leishmaniasis. The most important reservoirs of zoonoses and other parasitic infections are usually resistant or relatively insusceptible animals rather than highly susceptible ones. Plague and Chagas's disease illustrate this. Resistant and unduly susceptible animals occupy different “niches“ in nature. By “niche“ is meant the place of an animal in its biotic environment. Zoonoses often have a “focal distribution“ in nature. Thus wild rodents infected with plague may occur in “ pockets,” and the vector mites of scrub typhus congregate in “typhus islands.” An unstable environment often favours the transference of zoonoses to man and animals. Examples are the plague-infected plain of Rongai, in Kenya, the typhus-infected forest clearings in Malaya, and the yellow-fever infected forest edges in Uganda. Ecologically unstable areas are termed “ ecotones.” Zoonoses can also be transmitted in or near relatively stable sites such as huts, rodent burrows, caves, and termite hills. These are known as “habitat niches.” Animals and arthropods in a community are linked by food, and the importance of what are known as “food chains“ and “food cycles“ is discussed in relation to the transference of zoonotic infections from one host to another. Reference is also made to the fluctuations in numbers of various animals in nature, and how this affects the incidence of zoonotic disease. Certain highly susceptible rodents are periodically decimated by plague; this breaks the link with man, and human infections cease for the time being. © 1956, British Medical Journal Publishing Group. All rights reserved.