Although the fact that today viruses are known to 【S1】______ cause cancer in animals and in certain plants, that exists 【S2】______ a great reluctance to accept viruses as being of importance in human cancer. Basic biological phenomena generally do not differ strikingly as one goes from one species through 【S3】______ another. It should be recognize that caner is a biological 【S4】______ problem and not a problem that is unique for man. Cancer originates when a normal cell suddenly becomes a cancer cell which multiplys widely and without 【S5】______ apparent restraint. Cancer may originate in many different kind of cell, but the cancer cell usually continues 【S6】______ to carry certain traits of the cell of origin. The transformation of a normal cell into a cancer cell may have less than one kind of cause, but there is good reason to 【S7】______ consider the relationships that exist between viruses and cancer. Since there is no evidence which human cancer, as 【S8】______ generally experience, is infectious, many persons believe that because viruses are infectious agents they cannot possibly be of importance in human cancer. However, viruses can mutate and examples are known in which a virus that never kills its host can mutate to form. a new strain of virus that always kills its host. It does not seem unreasonable assume that an innocuous 【S9】______ latent virus might mutate to form. a strain that causes cancer. Certainly the experiment evidence now available 【S10】______ is consistent with the idea that viruses as we know them today, could be the causative agents of most, if not all cancer, including cancer in man. 【S1】