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What is TB?

Find out the basics of TB, where it comes from and where it goes. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Stanford School of Medicine.

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Video transcript

I've drawn out for you a mother over here, a mom, and her son on the right. And it turns out that mom has tuberculosis, let's assume that. And sometimes when you see "Tuberculosis" written out the way I'm writing it out, you'll actually see it shorthanded, or kind of use the quick way of saying it, which is two letters: TB. So let's say mom has TB. Now, this is actually a diagnosis, right? This is a description of her illness. This is telling us what she actually has, what she is sick with. But we have to remember that tuberculosis is actually caused by an organism. It's actually caused by a bacteria, it turns out. And this bacteria has the name "Mycobacterium". Mycobacterium Tuberculosis. So this is actually a very easy one to remember, because "tuberculosis" is right here in the name. Now I should point out Mycobacterium Tuberculosis is actually not the only cause of tuberculosis. It turns out there are few other kind of related mycobacterium, using this word "myco", that also cause TB. But this one, the one I wrote out for you, this is definitely the most common around the world, and that's the one I'm going to focus on. And in fact this "myco", this is actually Greek for the term "fungus". And the reason that this is here actually kind of tells us a little bit about how this bacteria grows, because it grows really slowly, like a fungus, and that's actually the reason that they use the term "myco". But nevertheless it is a bacteria, and so if we're going to put a little bracket around the diagnosis, I also want to put a little bracket around this part, to kind of distinguish the two. So now you can see very clearly TB, the diagnosis is caused by a bacteria. So now let's talk about how mom, who we said already is sick with TB... I'm going to actually just sketch out what her lungs might look like, assuming that the TB is in her lungs. This is actually the most common place we think of with TB, but not the only place. But let's say that she's got little red, I'm going to draw it in red, bacteria here in her lungs, causing her to be very, very sick with tuberculosis. She could spread it to her son. But what are the different ways that she might spread it? What are the most common ways? Well, let me sketch out a few possibilities, and we're going to go over whether these possibilities are very likely or unlikely to be a way for her to spread disease to her son. Let' say first they are sharing this delicious pizza I'm drawing here. Let's say they are very into pizza and they like to share food, and they both chow down on this little pizza here, that's one way they might potentially you might think of as a way to spread it. Maybe they're even sharing a drink. Maybe there is a drink here, and they are sharing again. You might also think about what's going on in their house: maybe they're opening and closing doors, and maybe they're touching door knobs, there is another way, right, maybe they're touching stuff in common. Maybe she says to him: "Hey, here, grab these keys" and she's been holding the keys all day, and then she gives him the keys and he holds the keys. There is another way, maybe the TB can touch objects in the environment, like a door knob or a key. And then there's the most obvious way you might be thinking: maybe she's coughing, maybe she has a loud cough, maybe she's coughing all day and some of these bacteria get in the air. That's another way that you might imagine that the bacteria could spread from her to her son. So different ways, right. Now, of these ways, I'm actually going to label this one over here, let's say this is through the air. Which are the most common ways to be really concerned about TB spreading? I'm actually going to just put it in green, so it really sticks out. The most common way is what we call "Person to person through the air". So in this case the first person would be mom, because she is sick, and it's going to go through the air, down to her son. And these other ways, for example food and drink, that's really not so common, that's really really unlikely to be a way of spreading TB and in fact, even this down here is really not likely either. So the idea of getting TB by sharing food and drink or touching objects in your environment like the keys or the door knob, or things like that, that's really not how TB spreads usually. Usually it spreads through the air. And one person, the sick person is usually coughing a lot, and then the other person might breathe it in. So let me make a little bit of space on this canvas and let's talk about what happens next. I'm going to draw one alveolus here, and I'm going to copy it a few times just so you can see a few different possibilities in terms of what might happen, and these represent the son's alveoli. These are the son's alveoli. And of course these are the tiny little air sacks at the very ends of the bronchial tree, right? So we'll make a few copies of this. There we go, we have four possibilities: Possibility 1, 2, 3 and 4. Basically, we'll go through different scenarios, different things that might happen when mom coughs. So maybe she coughs and the first possibility could be that the bacteria just don't get far enough, they don't actually make it to the son, and he never ends up breathing them in. So if this was the case, there would be no bacteria in his alveoli, of course his lungs are nice and clean, let me draw his lungs in, they look nice and clean, with no bacteria, and he's feeling great. Right, this is our son over here feeling really good, and we would say basically in this case, in scenario one, he's healthy, because the bacteria never even got to his lungs. Now, Scenario 2. Let's say that the cough actually was very strong and he was close by and he ended up breathing some of these in through his nose or his mouth and they went down into his lungs. That's another possibility. Once the bacteria get there, let me actually draw them on this little alveoli, in possibility number 2, they might actually get picked up by little immune cells. So he has little cells that are patrolling the lungs, making sure they are nice and clean and healthy, and these little immune cells, we'll label them over here, these are Macrophages, this literally means "Big eater", because "phage" means "to eat", so these immune cells, they might come by and gobble up these bacteria, and take them in, and destroy them. That's another possibility. So that would be possibility number two. So here the bacteria are gone. Now let's play it out again. And let's say in Scenario 3 also you have a couple of bacteria in here, and just as before, you got a couple of immune cells that come by, and they swallow up these little bacteria, these are the macrophages I'm drawing, swallowing up the bacteria, but let's say that unfortunately in Scenario 3, now, these macrophages, for whatever reason, cannot destroy the bacteria. The bacteria is still living, and that's why I draw them here as little red dots. They are still living, still there. And now let me draw the fourth scenario, which is again let's say a couple of bacteria get in, and the immune cells again get alerted, and they come by, and pick up one of them, maybe thîs immune cell is trying to go after this other one, maybe it's really close by, but here the key difference is that these bacteria are actually multiplying, so I'm going to draw lots of them. These bacteria are multiplying and they're filling up this space. So this space is filling up with little tiny red bacteria. So the key difference here is that these ones are multiplying. And we didn't really talk about the other scenarios having bacteria that are multiplying. But now, that's the key new thing here. And in this scenario, we'd call it "Active"... because you're actually seeing the bacteria thriving, we call this "Active TB infection". And that goes back to what we would label the other scenarios, these ones, and these ones together, we actually call both of them "Latent TB infection". And the reason I'm putting them together is because it's very hard clinically to distinguish Scenario 2 from Scenario 3, because in both cases, the immune system has previous experience with the TB bacteria, it's seen the TB bacteria, and in both cases you're not seeing lots and lots of bacteria dividing or multiplying, so we lump these together and call them both "Latent TB infection". The real key, and this kind of the take home that I want to point out, is that there is difference then between "healthy", someone that's really never seen TB in their life before; "Latent", where you have seen TB previously, but you don't have any bacteria that are multiplying; and "Active TB infection", where you have lots and lots of TB bacteria that are multiplying. Let me make just a little bit more space then. I'm going to focus now on just this final one, this multiplying, active TB infection situation. So if, let's say, our son in this case, gets tuberculosis from mother, from mom, and let's say unfortunately he has an active TB infection, what are some clues to tell us that he has an active infection? If I'm trying to figure out if somebody has TB, I always think about two key things: What are their symptoms? What are they sick with? That's the first thing. And then: How long is it going on for? I'm going to call that "Duration". And these two offer really really helpful clues to figure out if someone has TB. And with symptoms, I'm going to break it up into two categories. The first is "Constitutional", and this is constitutional symptoms, and this is the things that affect the whole body. The whole body, so I'm going to put a little bracket on the entire body to remind us of that. And this could be things like fevers or chills, you can't really point to one part of your body and say "This is the part that's having fevers and chills". You'd say just generally "I feel awful". This could be things like night sweats if you wake up and your t-shirt is all wet, you might say those are night sweats. Another example of a constitutional symptom is weight loss, particularly when you are not trying to lose weight, especially because you are maybe not eating as much, or you're vomitting. Anything like that... And now the other category is "Lower respiratory tract". "Respiratory" I'm going to abridge it to "Resp." tract. And this, if I want to draw it, it would basically be the part I've drawn in blue here. So going down from your voice box all the way to the alveoli. This would be your lower respiratory tract. And you can think about what sort of symptoms you might have there. It could be things like coughing, that would be coming from the lungs. If you're coughing very hard, you might have some blood or some little streaks of red that are blood in your sputum, so it could be bloody sputum. That would be another one. The sputum of course is just the mucus stuff that you cough up. And a lot of people that aren't coughing this much, they might have trouble breathing, or chest pain, anything like that. So these are just some examples of lower respiratory tract symptoms. And so I always think in my head: "Are they having constitutional symptoms?" If so, I put a check there. "Are they having some lower respiratory tract symptoms?" If so, I put a check there. And then "How long is it going for?" And usually with things like active TB infection, I'm thinking it's got to be usually more than 3 weeks. So more than 3 weeks. And this is again focusing on TB of the lungs, or the pleura, which is a space around the lungs, generally the symptoms have gone on for a little while. So these then become very helpful clues to figure out if someone actually has active TB infection.