British experts Sir Richard Doll and Richard Peto explain: “The various human cancers are diseases in which one of the many cells of which the human body is composed is altered in such a way that it inappropriately replicates itself again and again, producing millions of similarly affected self-replicating descendant cells, some of which may spread to other parts of the body and eventually overwhelm it.”​—The Causes of Cancer.

A cancer expert, Professor Stephan Tanneberger, stated: “It is now an established fact that this is a process involving several stages whereby a normal cell with a certain genetic make-up is transformed into a tumour cell under the influence of several factors. We know that viruses, radiation and chemical substances constitute such factors, but it is safe to say that only the interaction of several such factors produces a cancer cell in a multistage process.”​—Prisma.