Cancer is a very unsocial disease, being neither contagious nor infectious. (Diseases spread by direct contact are contagious; those spread through air or water are infectious.) Researchers, oncologists, nurses, caregivers or family never “catch” cancer, no matter how intimate their involvement with its victims.
Cancer is a micro-evolutionary process that works on the cellular level, as explained by biologist David Sloan Wilson:
Cell division is exceptionally well supervised. A new copy of DNA is made, like a monk transcribing holy text. The copy is proofread and corrected with such accuracy that the final error rate can be less than one in a million for any given letter of the text. Still, the entire text contains millions of letters, so most copies include a few errors. Many mutations have no effect on the cell’s function. Most of those that do are quickly detected and destroyed by the immune system.
In the case of cancer, enough mutants survive and adapt to cause havoc. Over many years and through thousands of iterations they morph and multiply until they destroy their host.
Every mutational “advance” that enables the cell line to avoid its “predators” (immune system), beat its “competitors” (the normally functioning cells), and colonize other areas of the body (metastasis) brings it closer to its own demise. Even if a tumor remains benign, it will wink out of existence with the natural death of the organism, lacking any mechanism for getting from one body to another.
Did you catch the glint of good news in that last sentence? For all its stubborn ferocity, cancer is myopic—and sterile.