If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:37

Video transcript

- [Voiceover] So we know a little bit about HIV now. It's a virus that attacks your immune system, and if untreated, it will cause a state of immune failure, immune deficiency in a person, and that immune deficiency state is referred to as AIDS. It's essentially late stage of an untreated HIV infection. So I just wanna talk a little bit about some of the ways that HIV is transmitted, some of the different ways you can possibly get HIV into your bloodstream and, thus, develop an HIV infection. And let's actually do this in a bit of a simplistic way, because it turns out that HIV is most often transmitted through contact with three different types, I guess, of body fluid from an HIV-infected person. So, contact with their blood, with their sexual or genital fluid, and I'll expand on what that means in a minute or so, and from breast milk contact. And let's actually look at this in terms of four fairly common scenarios. These will be the most common situations that HIV is spread via these groups of fluid here. And let me just make it clear that these infected body fluids would need to come into contact with broken skin or directly with your bloodstream, maybe via an injection or something like that, or they'd have to come into contact with one of your mucous membranes, which are just parts of your body that aren't protected by normal skin. So, for example, inside your mouth or inside the vagina or the rectum or at the opening of the penis. HIV can cross into your body through these mucous membranes. You won't just become infected by, say, just looking at these fluids or just being near them or anything like that. One of these fluids would have to make it inside your bloodstream or make it through one of your mucous membranes by physical contact for you to possibly develop an infection, okay? So, top left up here, we'll make this sexual fluid contact, which is actually the most common way that HIV is spread, typically when having unprotected sexual intercourse with someone who has HIV. And don't let these pictures scare you. These are just cross-sections of pelvises. So, a female pelvis on the left here and a male pelvis on the right here. So we can use these to look at the sexual organs. So, contact with sexual fluids like semen or preseminal fluid or vaginal fluid or even rectal fluid can spread HIV because there's often a fairly high amount of viral particles within these fluids, particularly within the first few weeks of a new HIV infection, when the infected person's viral load in their blood or in their sexual fluids is up at extremely high levels. But in terms of actual sexual activities, in general, unprotected anal sex with either a man or a woman is the highest risk sexual behavior, because not only is there exposure of HIV-infected sexual fluids to mucous membranes, right, the opening at the tip of the penis and the inside of the rectum in a woman or a man are mucous membranes, but if there's any trauma to the involved body parts, there can be some blood there as well, and that might have virus in it, and, thus, increase the risk of infection. Virus from infected sexual fluid or blood can get across the mucous membranes of an uninfected person, or it can just get directly into a person's bloodstream through an area that might be bleeding. Now this is not to say that unprotected vaginal sex is not risky as well. It turns out that unprotected vaginal sex is the second highest risk sexual behavior because, again, there's exposure of sexual fluids, right, semen and vaginal fluid to mucous membranes, which HIV loves to hop across when it can. And again, there could be, although less likely than in anal sex, there could be trauma to the involved body part. So there can be some blood involved as well, of course, containing the HIV. And just so that I'm perfectly clear here, anal sex, I said, had the highest risk of transmission and vaginal sex the second highest risk, but vaginal sex is the way HIV is transmitted most frequently overall, because overall more people have vaginal sex over anal sex. And so, last in this sexual-themed quadrant, I just want to mention that oral sex of any kind, so using the mouth to stimulate the penis, the vagina, or the anus, carries a risk of transmitting HIV for much the same reasons as before. Mucous membranes of the mouth being exposed to sexual fluids and mucous membranes of the penis or the vagina or the anus being exposed to possibly blood, maybe the person's mouth had some open sores in it, which happens to be one of the symptoms that can happen in an HIV infection. And on that note, actually, having untreated sexually transmitted infections, like chlamydia or syphilis or herpes, vastly increases your risk of transmitting or becoming infected with HIV in risky scenarios, because you're more likely to have breaks in your skin or your mucosal surfaces from symptoms of these STIs, like sores or ulcers, and through these, HIV can more easily infect you. All right, next quadrant here. So, after sexual transmission, right, the most frequent mode of transmission, exposure to infected blood. So that's the next most frequent mode of transmission, and in particular procedures involving needles contaminated by someone else's blood. So, for example, using intravenous drugs and then sharing needles with an HIV-infected person. In that case, HIV is just one of the many viruses you might pick up by sharing needles in IV drug use, because you're essentially transferring tiny droplets of an infected person's blood directly into your bloodstream on a needle. Other needle or related procedures, needlestick injury. Let's say you're a healthcare worker, and you're working with an HIV-positive patient, right, maybe you're drawing some blood or something like that, you might accidentally get stuck by the needle after it's been contaminated with their blood, and this does occasionally happen, unfortunately. There's a risk of HIV transmission from them to you there, and remember to keep in mind that risk of transmission depends on some extent to a person's viral load. So these risks are all sort of modified to be higher or lower depending on how much HIV's in a person's blood. Typically, very low amounts of HIV, if they're being treated with specific medications, and typically pretty high amounts of HIV, if they're not being treated. Last thing I'll say here is contaminated blood transfusions, and this isn't so much of an issue these days, at least not in most countries, because of some seriously rigorous screening of donor blood and the blood supply that blood banks have on hand, but back in the olden days, you know, before the sort of late 80s, early 90s, donated blood was not routinely tested for HIV, as crazy as that sounds. So, contaminated transfusions don't happen very often these days in most countries, but worth a mention here because it's still something to be aware of. Now third quadrant, what's the third most frequent route of HIV transmission? Well, it's actually from an HIV-infected mom to her baby during childbirth. And kind of alarmingly, around 90% of the HIV that we see in kids, they actually got it in this way, from their mom during their birth, which is also called vertical transmission, mom to child during gestation or during birth. And we're not 100% sure how the transmission happens, but the thought is just because they're being exposed to mom's blood and vaginal fluid on the way out of the birth canal, and because they, well, newborn babies are essentially all mucous membrane at birth. They just have really thin skin. So it's not too hard for HIV to jump on board, so to speak. Luckily, there's all sorts of medications that we can give to mom before and after her pregnancy and labor, and there is medication that the baby can take as well, and all of that drastically reduces the risk of mom passing HIV on to her baby, from a risk of about 25 to 30% HIV transmission without medication, all the way down to only about 1 to 2% with medications, so around a 20-fold reduction in mom's chances of passing HIV on to her baby with the proper medication. Now, last quadrant. This one also involves mom and baby. It's transmission of HIV via breast-feeding. Breast milk in an infected mom will contain HIV particles. So when the baby feeds, it'll be ingesting lots and lots of HIV particles, which obviously mom is not intending. And it's not clear how the HIV gets into the breast milk in the first place, but when the baby ingests it, it can infect the baby by being absorbed through the digestive tract. So before we wrap up, I just want to briefly mention two things. So first, the risk of transmission, in at least this quadrant here, it goes way down if the infected person is using condoms to sort of physically block HIV transfer and also properly taking their HIV medication to reduce the viral load in their body, in their blood and the fluids that we mentioned in this scenario here. But also that second one, the properly taking the HIV medications, that will vastly reduce the risk of transmission in needle-sharing and in mom-to-newborn transmission and in breast milk, all of these scenarios here. The second thing is that these are all ways you can contract an HIV infection, but I think it's worth mentioning a few ways you can't contract an HIV infection that might not be well known, or there might be some myths around these. So unless any of these has been contaminated by infected blood, HIV is not transmitted by body fluids like saliva or tears or sweat or urine or feces, and it is not spread through the air, like say tuberculosis is. So being around people with HIV won't somehow transmit the virus through the air to cause an infection in another person. And it's not spread from casual contact either, say from shaking hands with or hugging someone who has HIV. It actually doesn't even survive very well outside a human body. So you can't become infected by sharing plates with an infected person in a restaurant or touching a toilet seat at a public restroom or anything like that. But there you go, the main routes of transmission, unprotected sexual activity, contaminated needles, from mom to newborn, and via breast milk.