Carl Zimmer in his excellent blog, The Loom:
Malaria is caused by single-celled parasites called Plasmodium. A female mosquito carries them in its gut as it flies around in search of a victim to bite. After the parasites mature, they push through the insect’s gut wall, eventually making their way into its salivary glands. When the mosquito lands on a person and drills into the skin, it pushes some of its Plasmodium-laden saliva into the wound.
The parasites now begin their long journey through the human body. They get pushed by the surges of the bloodstream to the liver, where they invade cells and multiply inside them. The infected liver cells erupt with the next stage of Plasmodium’s life cycle, called merozoites. The merozoites end up back into the bloodstream, where they now invade red blood cells. They multiply yet again, rupturing the blood cells and invading new ones. Eventually the parasites achieve the next stage in their life cycle, when they’re ready at last to get sucked up by a hungry mosquito in a meal of blood.
If Plasmodium can’t get into a mosquito, all of this multiplication is for naught. So anything that the parasite can do to increase the odds of a successful exit can potentially be favored by natural selection. Last year, for example, a team of researchers found that mosquitoes were attracted to mice infected with Plasmodium parasites–but only when they were ready to leave their rodent host. The scientists found evidence that the parasites engineer this attraction by changing the odor of the mice. Infected mice give off odor molecules that draw mosquitoes to them.