Acquired immune deficiency syndrome (AIDS) is caused by a retrovirus called human immunodeficiency virus (HIV). When AIDS was first recognized, there were many newspaper reports on the possibility of mosquitoes being involved in its transmission.
There are three theoretical mechanisms which would allow blood-sU-Cking insects such as mosquitoes to transmit HIV.
 In the first mechanism, a mosquito would initiate the cycle by feeding on an HIV positive carrier and ingest virus particles with the blood meal. For the virus to be passed on, it would have to survive inside the mosquito, preferably increase in numbers, and then migrate to the mosquito’s salivary glands. The infected mosquito would then seek its second blood meal from an uninfected host and transfer the HIV from its salivary glands during the course of the bite. This is the mechanism used by most mosquito-borne parasites, including malaria, yellow fever, dengue, and the encephalitis viruses.
 In the second mechanism, a mosquito would initiate the cycle by beginning to feed on an HIV carrier and be interrupted after it had successfully drawn blood. Instead of resuming the partial blood meal on its original host, the mosquito would select an AIDS-free person to complete the meal. As it penetrated the skin of the new host, the mosquito would transfer virus particles that were adhering to the mouthparts from the previous meal. This mechanism is not common in mosquito-borne infections, but equine infectious anemia is transmitted to horses by biting flies in this manner.
 The third theoretical mechanism also involves a mosquito that is interrupted while feeding on an HIV carrier and resumes the partial blood meal on a different individual. In this scenario, however, the AIDS-free host squashes the mosquito as it attempts to feed and smears HIV contaminated blood into the wound. In theory, any of the mosquito-borne viruses could be transmitted in this manner providing the host circulated sufficient virus particles to initiate re-infection by contamination.
Each of these mechanisms has been investigated with a variety of blood sU-Cking insects and the results clearly show that mosquitoes cannot transmit AIDS. News reports on the findings, however, have been confusing, and media interpretation of the results has not been clear. The average person is still not convinced that mosquitoes are not involved in the transmission of a disease that appears in the blood, is passed from person to person and can be contracted by persons that share hypodermic needles.
There are a number of reasons why mosquitoes cannot transmit HIV.
Firstly the mosquitoes’ mouth parts do not operate like a hypodermic needle. The tube which injects the host with saliva is separate from the canal which the mosquito uses to sU-Ck blood from the same host. Therefore blood only flows into the mosquito and only saliva is injected; blood is not flushed out of the same canal.
Insect-borne diseases like Encephalitis and malaria are spread because they multiply within the mosquito, these diseases then move into the insect’s salivary glands and are injected into the host with the saliva. If a mosquito feeds on an HIV-positive person the virus cannot survive and replicate within the mosquito’s gut as HIV requires specialist cells found only in humans in order to multiply . HIV is treated as food and digested.
HIV circulates in the blood at lower levels than malaria and other inset-borne diseases. The mosquito does not take enough units of HIV from the infected person to initiate infection.
Even if it was possible for the mosquito to inject HIV into an uninfected person, the person would have to be bitten by ten million mosquitoes who had previously been feeding on an HIV positive host in order to receive one unit of HIV. For this reason squashing or accidentally swallowing a mosquito does not put one at risk of contracting HIV, because there is not enough HIV positive blood within the mosquito for a person to contract the disease.