Plasmodium is a genus of parasitic protozoans, four species of which cause malariain humans. Other species attack birds or lizards. In 1898 Ronald Ross demonstrated the existence of Plasmodium in the stomach of the Anopheles mosquito. For this discovery he won the Nobel Prize in 1902. However some credit must also be given to the Italian professor, Giovanni Grassi, who showed that human malaria could only be transmitted by Anopheles mosquitoes.
The four species of Plasmodium that attack humans are P. falciparum (the cause of malignant tertian malaria), P. vivax and the much rarer P. ovale (the causes of "benign" tertian malaria), and P. malariae (the cause of "benign" quartan malaria.)
The life cycle of Plasmodium is very complex. Starting with the cryptozoite stage hidden in the liver the parasite then invades the red blood cells as ring and amoeboid stages (trophozoites). Active parasites (merozoites) are produced which leave the red blood cells and travel within the blood to invade new red blood cells. After the merozoites infect new red blood cells, eventually the sexual stages (gametocytes) are produced (also in the blood), which are taken up by the mosquito and form zygotes which split to produce large numbers of tiny sporozoites. The sporozoites are the infective stage which migrate to the salivary glands of the mosquito where they are injected into the blood of the next host when the female mosquito bites. The sporozoites move to the liver where they repeat the cycle. The activities of the parasites in the blood, breaking down the cells and produce toxic materials that trigger the fever and chills of the disease. Because of the complex life cycle of these parasites it is difficult to develop a vaccine.
