 HIV is the virus that causes AIDS. The HIV virus enters cells of our immune system, uses the cell's machinery to make more virus, and then kills the cell. Current therapy for HIV is antiretroviral therapy, ART, which was established in the 1990s. These drugs have transformed HIV diagnosis from a death sentence into a manageable chronic illness. They work at various stages to prevent the HIV virus from making more virus and infecting more cells. Today, a major public health concern is reducing the risk of HIV transmission in order to manage the global AIDS epidemic. Antiretroviral therapy is a key player in this endeavor. While antiretroviral therapy is not a cure for HIV, it can reduce the amount of HIV virus present in the body, which we call viral load. Regular viral load testing helps measure how well antiretroviral therapy is working. When taken daily, effective antiretroviral drugs reduce viral load to a level below the threshold of detection for the test, which is currently 50 virus particles per milliliter of blood plasma. We call this undetectable viral load. It's important to understand that someone with undetectable viral load is not cured of HIV. They are still carrying the HIV virus. It's just present in such a small quantity in the plasma that the virus is not detectable using standard tests. Undetectable viral load is playing a huge part in managing the global AIDS epidemic by reducing HIV transmission with some words of caution. Here is why. As we just talked about, antiretroviral therapy can reduce the amount of virus in the blood to an undetectable viral load. This has two important consequences. The HIV positive person will be healthier since they are living with less HIV virus killing fewer cells in their immune system and causing fewer other illnesses. Because a person with undetectable viral load has less HIV virus, they are also less likely to transmit the virus to an HIV negative person. In two major studies with couples, having the HIV positive partner on ART led to a 92 to 96% reduction in the risk of HIV transmission to the negative partner. That's a very significant amount. It doesn't mean they can't transmit the virus. It just means that they are far less likely to do so. So undetectable viral load can also benefit HIV negative people. This is called treatment as prevention, where people with HIV take antiretroviral therapy to improve their own health and to decrease the chance of HIV transmission to a partner. Or on a larger scale, HIV transmission within a population. Multiple studies support the strategy for the health of both HIV positive and HIV negative partners and to reduce the number of new infections. However, undetectable viral load is not an absolute guarantee against HIV transmission. In fact, the amount of HIV virus present in the body can fluctuate over time if someone has a secondary infection, such as gonorrhea or chlamydia, or even without any known cause. These fluctuations could go undetected in between medical checkups. During those times when there is more HIV virus present, the individual may be more likely to transmit the virus, even though they had undetectable viral load at their last medical checkup. It's also important to know that viral load is not always consistent between an individual's plasma, semen and other genital fluids, and could potentially be higher or lower than the levels measured in the plasma. And of course, keeping the HIV suppressed depends on maintaining adherence to daily ART. Missed doses or breaks in treatment can cause viral load to rise. Finally, most data on treatment as prevention come from studies with heterosexual couples. We still need answers about how well undetectable viral load predicts HIV transmission risk in other contexts. For example, among gay and bisexual men, among people with a greater number of sex partners, and in settings where sexually transmitted infections are more prevalent. So to take away, HIV viral load is a major predictor of the risk for transmission of HIV. It makes sense, more virus present in body fluids, more virus available to enter a partner's cells and establish infection. Therefore, undetectable viral load is a central goal of antiretroviral therapy to maintain the health of HIV positive individuals. And is a promising new part of the HIV risk reduction toolkits, along with things like condoms and pre-exposure prophylaxis, which can protect against HIV. The global HIV epidemic is one of our century's biggest health concerns. Good medical care and treatment, effective prevention tools, and education are the most effective ways to ensure healthy lives for everyone. This video has been provided to you by Eureka Science in collaboration with Beta at San Francisco AIDS Foundation. To learn more about HIV treatment and prevention, visit beta at www.betablog.org. To stay in touch with Eureka Science, like us on Facebook, follow us on Twitter, subscribe to our YouTube channel, or visit us at www.eurekascience.com. Thank you for watching.