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.