Because insects live in almost all terrestrial and freshwater habitats, and eat almost every kind of organic matter, they are exposed to a great variety of diseases and parasites. The species that burrow in soil, or inhabit the mud around ponds, or feed on carrion or dung, are constantly subject to contamination by micro-organisms. Those that live among the litter and debris on the surface of the ground, or on the leaves and twigs of growing plants, run the risk of infection by air-borne microbes and of attack by many kinds of parasites.

To counter these dangers, insects have a number of features which operate to prevent infection. An obvious example is their exoskeleton, an almost complete covering of chitin and tanned proteins, physically tough, chemically resistant, and usually impermeable to water. It serves to exclude many potential contagions. Another example is the composition of their digestive fluids, which quickly destroy most of the micro-organisms that enter the alimentary tract with their food. These and other characteristics of their structure and physiology protect insects against many infective organisms.

In spite of defences of that sort, a large number of pathogens succeed in penetrating the insect body. Many kinds of viruses, bacteria, fungi, protozoa, parasitic worms, and parasitoid insects are able to invade the haemocoele, where they can nourish themselves on blood and whence they can find direct access to particular organs and tissues.