In studying cancer, as an amateur mind you, I've begun to think that you're not looking at a disease, at least not in 99 percent of the cases.  You're looking at a biological mechanism built into animal cells.  For the gene's interest, there is something that could go wrong with an animal phenotype-- tht is: the animal that cannot or will not reproduce itself, but which still competes for resources with copies of the same genes in the population.  To prevent this, a cell can "veto" the animal, sabotage it really.

When could this be triggered?  When the animal isn't having sex.  (Sex and reproduction may not be the same thing, but perhaps the cells can't tell the difference.)  Or, secondly, when many cells in the animal are genetically damaged.  I think when researchers begin to look closely, they'll find something strange.  Perhaps it won't be the damaged cells that become cancerous, but the normal cells around them.

/Zin