Cancers are serious conditions that are the cause of 25%-30% of all deaths. 1 in 2 people will have a form of cancer in their lifetimes. Cancers can have devastating long-term consequences beyond the illness itself, even if you survive. Treatment, recovery and sometimes remission is physically and mentally damaging.
But perhaps I'm the one who is wrong. Please, explain what you meant exactly so I can understand your perspective.
I am not the OP, but "cancer" is used as, and I quote, "(figuratively) Something damaging that spreads throughout something else". In that sense, the commenter could be implying, for example, that Apple products damage the electronic gadgets industry with their closed and anticompetitive policies, spreading silently (because users buy more of them to use the whole ecosystem).