<unk>  is caused by infection with parasites in the genus Plasmodium. In humans,<unk>  is caused by six Plasmodium species: P. falciparum, P.<unk> e, P. ovale curtisi, P. ovale wallikeri, P. vivax and P. knowlesi. Among those infected, P. falciparum is the most common species identified (~75%) followed by P. vivax (~20%). Although P. falciparum traditionally accounts for the majority of deaths, recent evidence suggests that P. vivax<unk>  is associated with potentially life-threatening conditions about as often as with a diagnosis of P. falciparum infection. P. vivax proportionally is more common outside Africa. There have been documented human infections with several species of Plasmodium from higher apes; however, except for P. knowlesi—a zoonotic species that causes<unk>  in macaques—these are mostly of limited public health importance. The life cycle of<unk>  parasites. Sporozoites are introduced by a mosquito bite. They migrate to the liver, where they multiply into thousands of merozoites. The merozoites infect red blood cells and replicate, infecting more and more red blood cells. Some parasites form gametocytes, which are taken up by a mosquito, continuing the life cycle. Parasites are typically introduced by the bite of an infected Anopheles mosquito. These introduced parasites, called "sporozoites", follow the bloodstream to the liver where they invade hepatocytes. They grow and divide in the liver for 2–10 days, with each infected hepatocyte eventually harboring up to 40,000 parasites. The infected hepatocytes break down, releasing this invasive form of Plasmodium cells, called "merozoites" into the blood stream. In the blood, the merozoites rapidly invade individual red blood cells, replicating over 24–72 hours to form 16–32 new merozoites. The infected red blood cell lyses, and the new merozoites infect new red blood cells, resulting in a cycle that continuously amplifies the number of parasites in an infected person. Over rounds of this infection cycle, a small portion of parasites do not replicate, but instead develop into early sexual stage parasites called male and female "gametocytes". These gametocytes develop in the bone marrow for 11 days, then return to the blood circulation to await uptake by the bite of another mosquito. Once inside a mosquito, the gametocytes undergo sexual reproduction, and eventually form daughter sporozoites that migrate to the mosquito's salivary glands to be injected into a new host when the mosquito bites. The liver infection causes no symptoms; all symptoms of<unk>  result from the infection of red blood cells. Symptoms develop once there are more than around 100,000 parasites per milliliter of blood. Many of the symptoms associated with severe<unk>  are caused by the tendency of P. falciparum to bind to blood vessel walls, resulting in damage to the affected vessels and surrounding tissue. Parasites sequestered in the blood vessels of the lung contribute to respiratory failure. In the brain, they contribute to coma. In the placenta they contribute to low birthweight and preterm labor, and increase the risk of abortion and stillbirth. The destruction of red blood cells during infection often results in anemia, exacerbated by reduced production of new red blood cells during infection. Only female mosquitoes feed on blood; male mosquitoes feed on plant nectar and do not transmit the disease. Females of the mosquito genus Anopheles prefer to feed at night. They usually start searching for a meal at dusk, and continue through the night until they succeed.<unk>  parasites can also be transmitted by blood transfusions, although this is rare. Recurrent<unk>  Symptoms of<unk>  can recur after varying symptom-free periods. Depending upon the cause, recurrence can be classified as either recrudescence, relapse, or reinfection. Recrudescence is when symptoms return after a symptom-free period. It is caused by parasites surviving in the blood as a result of inadequate or ineffective treatment. Relapse is when symptoms reappear after the parasites have been eliminated from the blood but persist as dormant hypnozoites in liver cells. Relapse commonly occurs between 8–24 weeks and is often seen in P. vivax and P. ovale infections. However, relapse-like P. vivax recurrences are probably being over-attributed to hypnozoite activation. Some of them might have an extra-vascular merozoite origin, making these recurrences recrudescences, not relapses. One newly recognised, non-hypnozoite, possible contributing source to recurrent peripheral P. vivax parasitemia is erythrocytic forms in bone marrow. P. vivax<unk>  cases in temperate areas often involve overwintering by hypnozoites, with relapses beginning the year after the mosquito bite. Reinfection means the parasite that caused the past infection was eliminated from the body but a new parasite was introduced. Reinfection cannot readily be distinguished from recrudescence, although recurrence of infection within two weeks of treatment for the initial infection is typically attributed to treatment failure. People may develop some immunity when exposed to frequent infections.
Malaria
Zika fever
Chikungunya
Leptospirosis
Airport malaria
Lassa fever
Brain fever
Dengue fever