It is likely that when considering the causes of disease you think first of infectious micro-organisms. You would be right to do so in that infectious disease plays a much bigger role in pig herds, particularly large intensive pig herds, than in animals kept individually such as dogs and cats. Nevertheless there are other non-infectious causes of disease which may also be damaging to the herd and to productivity and profitability. Causes of disease are considered here under nine main headings

Infectious agents

• Viruses
• Bacteria including; Chlamydia, Anaplasma and Mycoplasma
• Fungi
• Parasites

Non infectious agents

• Trauma
• Hereditary and congenital defects (developmental abnormalities)
• Nutritional deficiencies and excesses
• Toxic agents (poisons)
• Stress

There are other causes, such as tumours, which in pig herds are much less important and will not be dealt with here.

When you think of disease it is often only in terms of a single cause. In some cases this may be right (e.g. a poisoning or a highly virulent virus infection such as foot-and-mouth disease). In most cases you would be wrong however, because disease in pig herds usually results from the interplay between a number of predisposing, primary and contributory causes.

You should bear this in mind when you are thinking how to suppress disease.