Microbial infections of farm livestock may be either beneficial or detrimental to the host animal. Some components of the alimentary microflora fall into the former category and may play a major role in nutrition, particularly of the ruminant. 1 This contribution, however, will discuss only disease-producing microbes. Microbial disease has been broadly defined as any infection with bacteria, viruses, or protozoa that decreases the productive capacity of an animal. 2 In the present context, we are concerned with domesticated farm livestock maintained for food production, although infections of draft animals that have an essential role in agriculture will not be excluded.