The aetiology of Crohn's disease is unknown. Any hypothesis must take into account the continuing increase in incidence in some countries, including the UK. The increase affects the population from early teens, and specific comprehensive epidemiological studies in children and adolescents show a continuing rise in the rate of age and sex standardised incidence in Scotland.1 This suggests an environmental trigger, which may interact with underlying genetic susceptibility. A number of such environmental triggers have been proposed, including persistent infections, transient infections in a host with abnormal mucosal immunity, particulate materials, or dietary changes. Persistent infection with the measles virus after wild-type virus infections or immunisation with live attenuated measles vaccine have been proposed as important environmental triggers based on epidemiological observations.2 ,3 Measles is a single stranded RNA virus which can induce immune suppression. Measles infection is generally self limited and results in long term immunity, but rarely, persistent infection may occur after wild-type virus infection. The virus has special affinity for epithelial cells of the respiratory tract and cells of the immune system, such as lymphocytes and macrophages.4 A number of chronic diseases have been linked with persistent measles virus infection. These include multiple sclerosis, Paget's disease, a variety of autoimmune diseases, and autism, although definite evidence is lacking in each. Inflammatory bowel disease (IBD), especially Crohn's disease, has been linked to persistent measles virus infection. Ekbom et alproposed perinatal exposure to wild-type measles virus may lead to development of Crohn's disease later in life.2 ,3 Such case control epidemiological studies are of course prone to a variety of confounding factors, especially selection and recall biases. Epidemiologically robust data from a large controlled prospective study showed absence of a link between intrauterine exposure to measles and Crohn's disease.5 However, epidemiological …