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

DDT, or dichlorodiphenyltrichloroethane, was widely used as an insecticide to target mosquitoes, which are the primary vectors responsible for the transmission of malaria. The effectiveness of DDT in killing mosquitoes significantly reduced their populations in affected areas, thereby lowering the incidence of malaria transmission. This method of vector control was vital in malaria eradication efforts during the mid-20th century, as it directly addressed the source of the problem—the mosquitoes that spread the malaria parasite.

The other options are not accurate in terms of how DDT functioned in the context of malaria prevention. For instance, DDT does not kill the malaria virus; rather, malaria is caused by a parasite, not a virus. Additionally, while disrupting water sources can affect mosquito reproduction, that is not the mechanism DDT employs. Lastly, though DDT is aimed at the mosquitoes, it does not target the malaria parasite directly; its role is to eliminate the mosquitoes that carry the parasite. Hence, the use of DDT for killing mosquitoes was a critical strategy in combating malaria effectively.