Cancer is normally not infectious, but in a few animals tumour cells themselves pass between individuals like a parasite. Canine transmissible venereal tumour has spread among dogs for thousands of years, transferring during mating, and is effectively one cancer cell line that has outlived its original host. Tasmanian devils suffer from facial tumours spread by biting, which since the 1990s have devastated their populations. Some clams and other shellfish have contagious leukaemia-like cancers. These cases challenge the neat definitions of an individual, a disease and a parasite, since the cancer cells are a separate lineage living in other animals.

In 1909 D. W. Griffith made a 14-minute film about a speculator who corners the wheat market and doubles the price of bread. A farmer sows his field while a 'Wheat King' gambles in the Chicago wheat pit; bakers raise the loaf from five cents to ten and the poor line up for bread. Griffith cut back and forth between the farm, the feast and the breadline, a technique called parallel editing, so audiences saw both ends of the economy at once. The film drew on Frank Norris's stories and on a real scheme: months earlier Chicago financier James Patten had cornered wheat, sent bread prices up and made about $5 million, roughly $185 million today.