Malaria is a devastating infectious disease transmitted by mosquitoes and caused by infection with small one-celled parasites of the Plasmodium genus. The parasites destroy host red blood cells, resulting in anemia, fever, and chills. The illness is a driving force for human evolution and has changed the course of history on many occasions. Even today, with recent and profound understanding of the vector and the pathogen, the disease causes an estimated 2 million deaths every year; hundreds of millions more are infected with the parasite but do not die from the disease.
Bites from infected mosquitoes are no more bothersome than bites from uninfected mosquitoes. If enough malaria parasites are present to cause disease, patients usually experience malarial attacks that last several hours and feature fever, headache, muscular pain, and nausea. Between attacks, patients feel better but may become profoundly weak due to destruction of their red blood cells (RBCs). The attacks of chills and fever usually recur at around 48-hour intervals; symptoms caused by P. malariae recur at 72-hour intervals.
The large number of parasites and damaged RBCs in severe cases may cause problems with blood flow through small vessels and damage the kidneys or lungs. Parasites may also interfere with oxygen delivery to developing fetuses by damaging the placenta. A more severe form of the disease known as cerebral malaria results when damaged RBCs disrupt oxygen delivery to the brain; symptoms may include seizures, coma, or death.
The disease in humans is caused by only four species of Plasmodium parasites—P. falciparum (the most important), P. vivax, P. ovale, and P. malariae, collectively known as malaria parasites.
Infection occurs when the parasite gains entry into the blood through the bite of an infected mosquito of the Anopheles genus.The parasite has a multi-step life cycle, and various forms engage in specific behaviors that allow progress to the next stage. After the parasite enters the host, it travels in the bloodstream and reaches the liver, where it invades liver cells and multiplies. The infected liver cell eventually bursts, releasing a new form of the parasite back into the blood; this new form is able to enter red blood cells (RBCs) and feed on the hemoglobin molecules inside (a behavior known as hematophagy.) After a growth and incubation period inside the RBCs that may last several days, many more parasites are released, destroying what remains of the RBC. Parasites then travel the circulation (parasitemia) and may invade other RBCs or may be picked up by subsequent mosquito bites.
The incubation period, when parasites are devouring hemoglobin within RBCs and multiplying, may be asymptomatic; however, the parasites often mature and enter the bloodstream simultaneously in large numbers, causing profound fever and chills. The destruction of RBCs causes anemia, which can result in extreme weakness or death due to lack of adequate oxygen delivery to vital organs.
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