After four years of hiding his diagnosis from the public, Charlie Sheen announced Tuesday on the "Today" show that he is HIV-positive.
Sheen's doctor, Robert Huizenga, an assistant professor of clinical medicine at UCLA, accompanied him in the interview, saying that Sheen was treated immediately with strong antiviral drugs that have suppressed his "viral load" to "undetectable" levels.
But what does that mean?
When someone is infected with HIV, the virus hides out in their white blood cells and co-opts the cell's machinery to multiply inside them. When the cell eventually dies, it releases hundreds of HIV virus particles into the blood stream, which then goes on to infect other white blood cells and create even more virus. This kills even more white blood cells and damages the infected patient's ability to fight off other infections.
This impaired immune system due to HIV infection is a hallmark of AIDS, though Huizenga noted that Sheen does not have AIDS. AIDS status is determined by how many white blood cells are in a milliliter sample of the patient's blood.
If a patient like Sheen doesn't have AIDS, their HIV status is monitored by counting the number of viral particles in a patient's blood — what doctors call the patient's "viral load."
When this number is high, more HIV particles are present in the body, which usually means that either the immune system or the drug therapy are not doing a good job of fighting the virus.
Drugs called antiretrovirals were first approved in the 1990s. They block HIV's ability to make copies of itself inside white blood cells. These drugs help HIV patients keep their viral load down by interfering with HIV's ability to replicate. When the virus can't make as many copies of itself, it can't infect and kill as many white blood cells. This slows or stops the progression to AIDS.
This antiretroviral cocktail has also allowed HIV diagnoses to no longer be a death sentence, but to eventually become a manageable disease.
The treatment's goal is to bring the viral load to "undetectable" levels, which generally means less than 40 to 75 copies of the virus per one milliliter of blood, though this range of numbers can vary from lab to lab, according to AIDS.gov.
Having an undetectable viral load means that the patient is healthier and their immune system is in better shape. It also significantly reduces their chances of transmitting the virus to someone else. If less virus is in the body, there are fewer opportunities for the virus to hop to someone else.
But even an "undetectable" level doesn't mean that the virus is absent from other bodily fluids. It has been known to hide out in semen or vaginal fluids, even when it's undetectable in the blood.
The number of viral particles in the blood can also fluctuate between tests, according to AIDS.gov, and other STDs can actually boost the viral load in genital fluids, so it's important to practice safe sex, as the risk of transmitting HIV to others is still present.
Though researchers don't know exactly how much an undetectable viral load can lower the risk of transmitting HIV to partners, scientists are currently conducting large, multinational studies to find out.
Researchers from the National Institutes of Health reported in July 2015 that a decade-long clinical trial found that properly-used antiretroviral therapy is about 93% effective at thwarting transmission of the virus between heterosexual couples, where one is HIV-positive and the other is not. This association was only seen, however, when an HIV-positive patient was consistently taking the therapy and maintained a consistently undetectable viral load throughout the study.
In addition to being a proxy for health, the viral load test also clues doctors in to whether a patient's antiretroviral therapy is working. HIV can sometimes adapt to these medications by mutating its virus-copying proteins, which makes the treatment less effective.
But this therapy is not a cure. In fact, there currently is no cure for HIV, despite promising but still far-from-practical results from testing drugs, such as Truvada, that claim to prevent people from getting HIV in the first place.
