For many species of these bacteria, small animals (such as rats and mice) are the usual host. Cattle, sheep, or goats are the hosts for Coxiella burnetii, which causes Q fever. Humans are the usual host for Rickettsia prowazekii, which causes epidemic typhus. These animals and humans—the hosts—are called the reservoir of infection. Host animals may or may not be ill from the infection. Rickettsiae and rickettsia-like bacteria are usually spread to people through the bites of ticks, mites, fleas, or lice that previously fed on an infected animal. Ticks, mites, fleas, and lice are called vectors because they convey (transmit) organisms that cause disease. Q fever, caused by Coxiella burnetii, can be spread through the air or in contaminated food and water and do not require a vector. Each species of rickettsiae and rickettsia-like bacteria has its own hosts and usually vectors.