Causality in medicine: the case of tumours and viruses

Clarification of the aetiology of chronic human diseases such as atheroscle rosis or cancer is one of the dominant topics in contemporary medical resea rch. It is believed that identification of the causal factors will enable m ore efficient prevention and diagnosis of these diseases and, in some insta nces, also permit more effective therapy. The task is difficult because of the multistep and multifactorial origin of these diseases. A special case i n contemporary aetiological studies is definition of the role of viruses in the pathogenesis of human cancer. Virus-associated cancer develops only in a small minority of infected subjects, which implies that, if the virus do es play a role in the pathogenesis of the malignancy other factors must als o be involved. In this paper the author attempts to review the present meth odological approaches to aetiological studies of chronic diseases, discusse s the role of criteria for identifying causal relationships and proposes gu idelines that might help to determine the role of viruses in human cancer.