资料题：According lo the passage, cancer cells are（　）。

Despite the fact that today viruses are known to causes cancer in animals and in certain plants.there exists 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 to another.It should be recognized that cancer is a biological problem and not a problem that is unique for man.Cancer originate when a normal cell suddenly becomes cancer cell which multiplies widely and without apparent restraint.Cancer my originate in may different kinds of cell, but the cancer cell usually continues to carry certain traits of the cell of origin.The transformation of a normal cell into a cancer cell may have more than one kind d cause, but there is good reason to consider the relationships that exist between viruses and cancer.Since there is no evidence that human,as a generally experienced, 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 assume that an innocuous latent virus might mutate to form a strain that causes cancer.Certainly the experimental evidence now available 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.