Two other arboviruses, causing yellow fever and dengue, are exotic to Canada, occurring only in individuals that have recently returned from the tropics. Yellow fever used to occur in late summer along the eastern sea board, when boats carrying populations of the mosquito vector, Aedes aegypti, and presumably also infected seamen, sailed north from southern harbors. The vector can breed in any water-filled container, and does particularly well in bilge water in wooden ships. The advent of modern steel vessels, along with efficient mosquito control, eliminated this hazard from North America. The lack of suitable vector mosquitoes in Canada precludes an epidemic of either of these diseases that might otherwise result from a returning infected tourist.
Malaria is a disease caused by a protozoan.
Unlike viral infections, protozoa seem more resistant to the body's defense mechanisms, and once malaria is contracted, symptoms may occur again and again if not treated. Although various animals, including birds, lizards, and monkeys, may contract various kinds of malaria, human malarial organisms have no host other than man and therefore the disease cannot be contracted away from human habitation. It is transmitted biologically only by species of the mosquito genus Anopheles. At least five of the six species of Anopheles found in Canada have transmitted malaria experimentally. Transmissions of malaria occurred recently among campers in California, because one of them had the disease.