I heard that mosquitos are attracted to people with A+ blood. Is this true, and if so, why?

This is probably one of those unpteen urban myths that float around. There is some anecdotal evidence that mosquitoes are attracted to people with different blood types, but which blood type attracts them varies with the story. Some claim that females better attract mosquitoes than males, but this also appears to be a myth.

Female mosquitos use a number of environmental clues to identify a target: body heat, lactic acid, CO2 (carbon dioxide), water vapour, and numerous other chemicals produced by the human body (e.g., sebum, sweat).

Visual stimulation also plays a roll. A mosquito that feeds during the day - different species feed at different times - can identify a target by its motion and are more strongly attracted to dark-coloured clothing than light. Once the mosquito comes closer, it makes a more precise identification using smell. Different species may also bite different areas of the body, depending on what combination of environmental clues they use. Perfumes, used in a range of body care products (e.g., shampoo, skin cream), also help mosquitoes identify a target.

In general, men are bitten more often than women, and both more frequently than clidren. This is likely related to differences in the physical size of humans, with larger persons providing a larger heat source and producing more CO2. Ultimately, it is your unique smell that draws a mosquito in for lunch.

Mosquito repellants work by interfering with the female's ability to use environmental clues to detect a target. Repellents may be applied to your skin directly or control devices are used to mask clues in the general vicinity or draw the mosquitoes away. The latter group includes 'bug-zappers', which use a combination of UV light, CO2, and octenol (a natural organic compound) to attract mosquitoes to an electric grid, citronella candles, and the mosquito plant. The most effective control agent is DEET, a finding confirmed in numerous studies.

I think that mosquitoes are attracted to any type of blood because as long as they have blood in their body they will live. ;)