Mosquitos and man, a lethal cycle at work since the dawn of time

It begins and ends with a hungry mosquito, settling on human skin to suck up its fill of blood and unwittingly completing the life cycle of a parasite that, in one of nature's quirkiest three-way relationships, depends upon both insect and human being for its existence.

And not just any mosquito: the carrier - or "vector" - is always an Anopheles mosquito - or, at least, one of the 20 out of the 380 species of Anopheles mosquitoes capable of transmitting malaria. Male mosquitoes do not feed on blood; it is always a female, and usually between dusk and dawn. These are the vampires of the insect world.

The disease we call malaria (an 18th-century contraction of the Italian for "bad air"; marsh gases were originally blamed for causing the disease) - exists in a vicious cycle in which both human and mosquito play their parts as hosts, but in which only humans pay for the privilege with their lives. The mosquito takes in cells of the Plasmodium parasite from an infected human, fertilises them in its gut and returns the resultant spores to another human being via its salivary gland.

This relationship, say experts at the Department of Microbiology and Immunology at the University of Leicester in England, has been going on "since the dawn of time". The parasites probably originated in Africa, along with man, and mosquito fossils have been found dating back 30 million years. In another one of nature's quirks, "the Plasmodium parasites are highly specific, with man as the only vertebrate host and Anopheles mosquitoes as the vectors. This specificity of the parasites also points towards a long and adaptive relationship with our species".

There are four species of Plasmodium; one of the most common, and the most deadly, is falciparum, common throughout the World Health Organization's Eastern Mediterranean region, which includes Arabia, Iraq and Iran.

In the human host, the parasites head for the liver, where they multiply and infect red blood cells, so threatening the blood supply to vital organs. According to the WHO, symptoms might not appear for 10 to 15 days, when the victim may start to experience headaches, fevers and vomiting - symptoms often mistaken at first for flu - and, "if not treated within 24 hours, P. falciparum malaria can progress to severe illness often leading to death".

Caught sufficiently early, malaria can be treated with a number of different drugs but, says the WHO, because the parasites quickly develop resistance, the best solutions are personal protection - such as prophylactic antimalarial tablets and sleeping nets - and "vector control", chiefly, the elimination of mosquito breeding grounds, such as pools of stagnant water, and the use of insecticides.