Before the automobile, land transportation by means other than the human foot was chiefly by animal (by horse, mostly). (Before the railroad, which debuted in the early 19th century, the only means of land transportation was the human foot or animal power.) The emissions from horses and other transportation animals were themselves unpleasant and toxic. These emissions also attracted flies and other insects. Flies feasting on the mounds of transportation emissions that fouled city streets would spread bacteria from these emissions directly to humans and onto human food and into human drink.