"an impromptu lecture on cancer of the colon or tapeworms"
Colon cancer is more aptly termed colorectal cancer, and usually arises from polyps in the colon. Polyps are fleshy growths that if left untreated can develop into cancer of the colon.
The tapeworm is a curious specimen. It can be transferred to human bowels when the meat of an animal, such as a cow, is consumed. They are a type of parasitic flatworm, and can grow as long as 12 metres! The worm anchors itself in the gut of the host, growing a long tail made up of several segments. Every segment is independent and has its own digestive system. The segments are discarded as they grow older, and only the reproductive systems are left - it is basically a sack of tapeworm eggs. The tapeworm survives in the host by absorbing food through its skin as it is being digested by the host.