Mosquitoes are found all over the world, except in Antarctica. These two-winged insects belong to the order Diptera. Members of the genera Anopheles, Culex, and Aedes are most commonly responsible for bites in humans. There are approximately 170 species of mosquitoes in North America alone.

To develop, mosquitoes require an environment of standing water. As a group, they have adapted to complete their life cycle in diverse aquatic habitats, including fresh water; salt water marshes; brackish water; or water found in containers, old tires, or tree holes. The life cycle of the mosquito has four stages. The female mosquito lays her eggs, up to several hundred at a time, on the surface of the water or in an area subject to flooding. Unhatched eggs of some species can withstand weeks to months of desiccation, remaining viable until the right conditions for hatching occur. The eggs of most species hatch in 2 to 3 days, and the larvae feed on organic matter in the water for about a week until they change into pupae. The pupae live at the surface of the water for 2 to 3 days before metamorphosing into adult mosquitoes.

Only female mosquitoes bite. Male mosquitoes feed primarily on flower nectar, whereas female mosquitoes require a blood meal to produce eggs. They usually feed every 3 to 4 days; in a single feeding, a female mosquito typically consumes more than its own weight in blood. Certain species of mosquitoes prefer to feed at twilight or nighttime; others bite mostly during the day.

Some mosquito species are zoophilic (preferring to feed on animals) and others are anthropophilic (showing a preference for human blood). In some mosquito species, seasonal switching of hosts provides a mechanism for transmitting diseases from animal to human. (It is worth noting, however, that mosquitoes cannot transmit HIV because the virus neither survives nor replicates in mosquitoes and the blood from the last bitten person is not flushed into the next person during subsequent feeds. In addition, the circulating viral load of most HIV-infected persons is so low that the theoretical risk that a mosquito bite would transmit HIV is estimated to be less than 1 in 10,000,000.)