The increase in CD that has taken place over the last 50 years has paralleled the increasing use of antibiotics in human and veterinary medicine. Insufficient doses of antibiotics can induce a capacity for toxin production in bacteria, or can make them invasive. Prior antibiotic therapy also promotes the outbreak of CD to a statistically demonstrable extent. It may clearly be assumed that CD is caused in genetically susceptible persons by intestinal bacteria whose biological (but not morphological) properties have been altered by antibiotics.