In affluent societies the prevalences of so-called 'Western' diseases such as atherosclerosis, allergies and autoimmune disorders appear to have increased, while many diseases caused by communicable infections are now relatively less common. To test whether there may be a causal relationship we examined the effects of Schistosoma mansoni infections in mice that develop cardiovascular pathology as a result of a genetic deficiency in apolipoprotein E (apoE-/-). The development of atherosclerotic lesions in the aortic arch and brachiocephalic artery of the apoE-/- mice was reduced by approximately 50% in mice with the parasitic infection, when comparison was made with uninfected control mice fed the same diet. Observations on S. mansoni-infected conventional laboratory mice indicate that patent schistosome infections could be counteracting the effects of an atherogenic diet by modulating host lipid metabolism and inducing a reduction in blood total cholesterol concentrations.