What is Malaria?

Malaria is a parasitic infection transmitted to humans through the bites of infected female Anopheles mosquitoes. The resulting disease in humans can be devastating. After spreading rapidly through the bloodstream to the liver, the parasite emerges again into the blood stream, finally to settle in the red blood cells, where it multiplies and emerges in bursts of new organisms. These parasites, because of their large numbers, can cause particular damage to the nervous system, liver, and kidney. In young children and adults who have not recently been infected (and therefore have not developed natural immunity), this cycle can result in death within hours from cerebral malaria. Others die later in the infection from overwhelming anemia or liver and kidney failure. Untreated, up to 20% of persons infected with falciparum malaria will die. Four species of malaria infect humans, though only two, falciparum and vivax cause the vast majority of clinical cases and nearly all of the deaths and serious morbidity. The disease is particularly prevalent in sub-Saharan Africa, and is common throughout tropical regions of China, India, Southeast Asia, and South and Central America. Almost all of the serious morbidity caused by falciparum malaria occurs in children under the age of ten, and the impact is especially severe in those under the age of five. Protecting these children from malaria is a major goal of current malaria vaccine development efforts. Malaria has a complex life cycle. Infected female mosquitoes inject malaria sporozoites when they bite, and the sporozoites are carried to the liver where they rapidly infect liver cells. Without causing symptoms, these sporozoites undergo a radical change and multiply furiously for the next 4-5 days. Tens of thousands of asexual stage merozoites are released from each infected liver cell, each of which rapidly target and invade a red blood cell. Every few days, the merozoites multiply ten-fold and burst out to infect other red blood cells. This cyclic and massive increase in parasite burden gives rise to the clinical disease we recognize as malaria. In the absence of immunity or drug treatment, death can occur within hours of noticeable symptoms. If death does not occur and infection continues, some of the parasites further differentiate into a form that is infectious for mosquitoes, thus permitting the life cycle to continue. Strategies for developing malaria vaccines have been targeted at specific points in the parasite life cycle during which the organism appears particularly susceptible to the host's immune system.