Slap! Another mosquito! I try to resist the urge to scratch, but it would be easier to refuse a glass of water on a 110-degree day. I scratch, and oh, glorious relief! The feeling is just momentary, though, because here comes that hot sensation, and now my skin is swelling into a hideous red bump. Who knows what disease that thing could be carrying? At least I can be sure it isn’t HIV.

Scientists have pretty much ruled out the possibility that mosquitoes can spread the virus that causes AIDS. No documented case of HIV has ever been linked to the hated bloodsucker. While lack of evidence cannot by itself disprove a hypothesis, the chances of a mosquito transmitting HIV are so slim that the idea has faded out of scientific discussion as researchers face the real challenges of the immense predicament of AIDS.

However, when scientists were first learning about HIV, the insect transmission question was yet another unknown about the new disease. Some experiments and unexplained cases in the 1980s led to finger-pointing at mosquitoes, although scientists already had strong doubts that insects could transmit the disease.

In 1987, the now-defunct U.S. Office of Technology Assessment held a workshop to address concerns about a possible HIV threat from mosquitoes, bedbugs, ticks and cockroaches. Besides room for “a rare and unusual event” of possible insect transmission, the report states that it is almost impossible for the insects to pass along HIV.

The discussion has almost fizzled out, although a few investigations scattered over the years have continued to look for connections between HIV transmission and insects such as bedbugs and flies. In 2006, the United States Army Center for Health Promotion and Preventive Medicine issued a definitive report that outlined why there is no reason to worry about contracting HIV from a mosquito bite.

But why can’t you get HIV from a mosquito when it’s clearly the culprit in malaria, yellow fever and dengue fever? It’s all about the bug. There are two methods by which bloodsucking insects typically transmit disease: the biological method and the mechanical method.

The biological route is how malaria infects more than half a billion people each year. Its disease agent, the Plasmodium parasite, relies on the mosquito as a go-between to settle in human hosts.

Every mosquito bite involves a female mosquito looking for a blood meal to nourish her eggs. She injects saliva to keep the blood from clotting, and an allergic reaction to the saliva makes our skin annoyingly itchy and red after the bite. If the mama mosquito happens to bite a malaria-infected person, she ingests the parasites, which end up invading her cells and replicating. They then migrate to the salivary glands from where they can infect another human host in her next bite.

If the blood that she sucks up contains HIV, though, the virus can’t follow the same path as the malaria parasite. Instead of multiplying and eventually heading for the salivary glands, the viruses get digested, and meet their death in the insect’s gut.

The mechanical method is the other way for bloodsucking insects to pass along disease. Suppose a feeding mosquito is slapped away but is still hungry. Since insects don’t use napkins, blood remains on its mouthparts as it flies over to bite another victim. Theoretically, if Victim 1 had HIV circulating in his bloodstream, some could end up in Victim 2.

However, the probability of the transaction is almost zero. For one thing, the mosquito needs a healthy victim within quick buzzing distance of the HIV-positive one. Even in these conditions, the mosquito’s eating habits and the nature of HIV’s presence in the bloodstream still make it difficult to pick up viruses to transmit.

In a typical meal, a mosquito eats just a thousandth to a hundredth of a milliliter out of the average person’s 5.5 liters of blood. That’s like drinking a two-liter soda bottle of water out of an Olympic-sized pool.

From its tiny snack, the mosquito has hardly a chance of ingesting HIV. While the amount of the virus in blood varies from a few dozen to several hundred thousand viruses per milliliter, usually the levels are low. Blood left on the sloppy mosquito’s mouth is highly unlikely to have any HIV in it. If the mosquito bit someone with 1,000 viruses per milliliter, for example, there would be a 1 in 10 million chance of injecting just one virus body into another victim.

By now, scientists have a clear understanding of the ways HIV is spread, and insects are not one of them. With HIV’s estimated annual cost of around $20 billion and immeasurable effects on its victims, we’re lucky that the pesky mosquito’s bite isn’t another weapon in the disease’s arsenal.