They are transmitted to humans by mosquitoes in the genus Anopheles . When a mosquito bites an infected individual, the parasites are taken up in the blood. They reproduce sexually in the gut of the mosquito, and work their way to the salivary glands. Once there they can be transmitted to the next human host. Mosquitoes use their saliva to prevent blood from clotting, so some of the salivia, carrying Plasmodium , mixes with the victims blood, effecting transmission.
Malaria is a disease of the agriculture revolution.
Clearing of forests, and people living in larger communities contributes to the ease of spread. In Africa, where P. falciparum is most prevalent, over 750,000 people die each year of malaria. In the late 1940's population geneticists began to see a pattern. The geographic patterns of abundance for sickle-cell anemia and malaria were very similar. A recent map shows this pattern.