In a bold experiment, scientists purposefully exposed people to mosquitoes carrying malaria parasites. At the same time, they were also given a dose of a drug called chloroquine. After three months, these people didn't develop the disease at all, even after they stopped taking the drug.
The chloroquine protected the subjects from the feasting mosquitoes, while the malaria exposure strengthened the humans' resistance to it. At the end of their experiment, all ten subjects exposed to malaria had built up immunity to it. The body's own immune system fought the parasite in its early stage, while the drug attacked it in the later stages.
It is not clearly stated and seems unlikely that the immunity could be achieved in the long term in the face of continuing repeated exposure, but it helped scientists identify a specialized group of cells that guard against the malaria parasite. Their results were much more successful than other vaccines currently used in experiments.