Malaria has been one of the world’s worst killer diseases throughout recorded human history. Despite attempts to eradicate it, it remains one of the worst diseases in terms of deaths annually, and has actually increased in incidence since the 1970s. About 200 to300 million new case occurs worldwide each year and about 1.5 million deaths over two third of which occurs in Africa. It is particularly common in Africa south of the Sahara and is widespread through Asia and Latin America. It is used to be common in Europe and North America (Oliver Cromwell died of malaria and Sir Walter Raleigh suffered from it.). Malaria provides a good example of how social, economic and biological factors are all important in controlling disease.

Transmission, signs and symptoms
The causative agents of malaria are the protozoan parasite Plasmodium. Four species cause malaria, but most cases are caused by one of two species. Plasmodium vivex is found in the subtropics; Plasmodium falciparum is more common and more lethal, and is found in both the tropics and subtropics. It is the largest single cause of death in Africa. Malaria is transmitted by female mosquitoes of the genus Anopheles. This mosquito is itself a parasite, the females visiting humans for occasional meals of blood. During feeding, infected mosquitoes pass on the malaria parasite from their salivary glands. The mosquito is described as a vector.

An immature form of Plasmodium is injected into the blood of humans by the mosquito. This form disappears from the bloodstream as it enters various cells of the body, particularly the liver. Here it multiples to produce large numbers of a form (the merozoite) which can infect other liver cells. Finally it leaves the liver and enters red blood cells. Each parasite cell in a red blood cell undergoes further division. The red cell bursts and the released parasite cells can enter other red blood cells. As a result of this extensive division, millions of these parasites can be present in the blood. Some of the parasites transform in to male and female forms of the parasite (gametocytes).

If infected red blood cells are now sucked up by a mosquito the parasites survive digestion in the stomach of the mosquito and transform into male and female gametes. Fertilization occurs and the zygote penetrates the wall of the stomach where it grows to produce a swelling containing immature parasites.

People who have been exposed to infection since birth and who have survived attacks of malaria develop a certain amount of tolerance to the disease. People with no history of previous infection will develop serious disease very rapidly. Ten days after infection a few fever develops and the body temperature increases rapidly to 40.6-41.7ºC. The fever may last as long as 12 hours accompanied by headache, generalized aches and nausea. After the fever sweating starts and then the temperature falls. The area of the abdomen over the spleens is tender.

The symptoms and fever coincide with the multiplication of the parasite when the red blood cells burst open and the parasites are released. The fevers occurs every third day in P.vivex and P.falciparum malarias. The attacks can be complicated, however, as a result of successive infections by mosquitoes. The infections by P.falciparum cause malignant malaria in which the fever is accompanied by other complications. The parasite tends to accumulate in blood vessels in the brain, causing convulsions or coma. Other common complications include kidney failure and pneumonia. Malaria caused by P.falciparum can be fatal within two or three days.

Prevention and treatment
Prophylaxis that is the use of medicines (drug) to prevent disease can be used by people entering areas endemic for malaria. The usually accepted prevention is chloroquine or mefloquine which is taken weekly before and during a visit to endemic areas. The dose must also be taken for six weeks after leaving the area. Other synthetic drugs are proguanil hydrochloride and pyrimethamine. The effectiveness of these drugs has declined as the parasite developed resistance. P.falciparum in now resistant in America, East Africa and S.E. Asia. New drugs are constantly being sought, but pharmaceuticals companies are tending to scale down work on ant malarial drugs because there is little profit to be made from developing countries.

A drug called Fansidar and another called Lariam (mefloquine) are effective against chloroquine-resistant Plasmodium. Mefloquine can cause unpleasant side effects such as nausea, dizziness, diarrhea, vomiting, headaches, abdominal pain and occasionally psychiatric disturbance. It was introduced in 1985 but already half the malaria cases in Thailand are resistant to it. A traditional drug, quinine has come back into favors, but resistance is also growing to this.
Together with the use of drugs, other measures such as clothing (long sleeves trousers) and mosquito nets at night should be used in mosquito area. These are now often impregnated with new insecticides. It may be necessary to administer drugs by injection. This then produces a high concentration of the chosen drug to kill the malaria organisms in the blood.

Eradication of malaria
In 1955 the World Health Organization launched a programs designed to eradicate malaria from the world. Large sums of money were devoted to this project and the methods employed were as follows:
Drainage of stagnant water
Destruction of the breeding stages of the mosquito
Destruction of the adult mosquitoes.