Medical entomology is the study of insects, spiders, ticks, and mites, collectively referred to as arthropods, and the diseases they transmit. Military medical entomology began in 1900 when Major Walter Reed and his colleagues demonstrated the transmission of yellow fever by mosquitoes and were subsequently able to reduce the impact of this disease on the U.S. military. From typhus in World War I, malaria in World War II, to leishmaniasis today in Operation Iraqi Freedom, military medical entomologists have been called upon to protect the military mission. The United States Air Force currently has 17 active-duty and 6 reserve medical entomologists working around the world.