Malathion is the name of an organophosphate insecticide used in Vietnam to combat mosquitoes. Organophosphates were first developed in the late 1930s by Nazi Germany as chemical warfare agents (nerve gas). VX nerve gas and Sarin gas are well-known examples. Organophosphates operate on humans and insects in a similar fashion by attacking the nervous systems.
During the Vietnam War, large numbers of troops came down with malaria. To kill the mosquitoes that carried the disease, the Department of Defense converted aircraft that were spraying Agent Orange to the spraying of Malathion. This was the start of Operation Flyswatter.