Malaria isn't in the country because in the late 40s the US Public Health Service sprayed the interiors of five million southern homes with DDT, drained wetland

Malaria isn't in the country because in the late 40s the US Public Health Service sprayed the interiors of five million southern homes with DDT, drained wetlands, and carpet bombed counties with pesticides. They did that until the mosquito population dropped enough that the transmission cycle was broken. There are still tons of mosquitoes able to carry malaria in the US. They haven't been killed off - heck, my cat caught one in the laundry room a few days ago! But humans are the only relevant reservoir for malaria, so as long as we use good public health techniques like screening individuals from endemic areas, it's possible to let the US remain malaria free.