根据下列短文，回答题。
　　Despite the fact that today viruses(病毒)are known to cause cancer in animals and incertain plants，there exists a great reluctance to accept viruses as being of importance in humancancer．Basic biological phenomena generally do not differ strikingly as one goes from one spe—cies to another．It should be recognized that cancer is a biological problem and not a problemthat is unique for man．Cancer originates when a normal cell suddenly becomes a cancer cellwhich multiplies widely and without apparent restraint．Cancer may originate in many differentkinds of cell，but the cancer cell usually continues to carry certain traits(特性)of the cell of origin．The transformation of a normal cell into a cancer cell may have more than one kind of cause．but there is good reason to consider the relationships that exist between viruses and cancer．
　　Since there is no evidence that human cancer，as generally experienced，is infections，manypersons believe that because viruses are infections agents they cannot possibly be of importancein human cancer．However，viruses can mutate(突变)and examples are known in which a vi．rus that never kills its host can mutate to form a new strain of virus that always kills its host．Itdoes not seem unreasonable to assume that an innocuous(无害的)latent(潜伏的)virus mightmutate to form a strain that causes cancer．Certainly the experimental evidence now available isconsistent with the idea that viruses as we know them today，could be the causative(引起……的)agents of most，if not all cancer。including cancer in mall．
Cancer should be considered to be a biological problem rather than a medical one because_________．