A special case in contemporary etiological 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 does play a role in the pathogenesis of the malignancy, other factors must be also involved. The author summarises the general properties of human tumour viruses, reviews the development of causal thinking in microbiology and proposes guidelines that might help to determine the role of viruses in human cancer.

        Key words: role of viruses, malignant tumors.