Insanitary penning conditions, particularly for domestic animals, may play an important part in the spread of animal disease, particularly as many infections are transmitted through faeces and other excreta. Coccidiosis and foot-and-mouth disease are encouraged by the existence of insanitary conditions.
The environment of factory farming provides ideal conditions for both the existence and expansion of several animal diseases. Because of the large numbers of animals reared in one place, disease can completely destroy a farm's stock or the stock of an area. Diseases may be spread through their virulence in intensive farming units, or through inadequate hygiene measures taken in the disposal of diseased animals or in the sterilizing of implements which have come into contact with them.
Diseases such as Newcastle disease in poultry are particularly well adapted to factory farming conditions. The quantity of animals in a small space means that the disease can spread very rapidly. Animal stress caused by close confining, the need to clip the beaks of poultry to check cannibalism, and general weakness caused by living in unnatural conditions and by being made to produce at an abnormally high rate, leave animals susceptible to disease and with very little resistance. Intensively farmed animals tend to be specially bred for particular qualities and have uniform characteristics, which means that disease can spread more rapidly. Intensive farming units act as a trap for airborne animal diseases and mechanical ventilation ensures that infected particles circulate through the unit many times. Other contributors to the spread of animal disease in factory farming units may be overcrowding and unsanitary conditions.