What was DDT widely used for in some countries to combat malaria?

DDT, or dichloro-diphenyl-trichloroethane, was widely used in some countries as an insecticide specifically to target mosquitoes, which are the primary vectors for malaria transmission. Malaria is caused by parasites that are carried by female Anopheles mosquitoes. By effectively killing these mosquitoes, DDT significantly reduced their populations, consequently lowering the incidence of malaria in affected areas.

The strategy focused on vector control, as controlling the population of mosquitoes can directly impact the transmission dynamics of the disease. This approach has proven effective in many regions, allowing for better health outcomes and reducing the burden of malaria, especially before concerns regarding DDT's environmental and health impacts became more prominent.

The other options do not accurately reflect the role of DDT in combating malaria. The virus or the parasite causing malaria is not what DDT targets; instead, it specifically affects the mosquitoes that carry and transmit these pathogens. DDT's intended use was to prevent the spread of malaria by controlling the mosquito population, making the choice that highlights its role in killing mosquitoes the correct answer.