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- [Voiceover] Infectious arthritis is where you have an infection in the joint, usually bacterial. And we think of how a joint is one bone then the other bone, some cartilage in the middle and with this joint capsule that closes off the space. Usually the synovial fluid in here is sterile. This is not open to the environment, and it's sterile and clean in there. And an infectious arthritis happens when the bacteria gets in there and causes an infection, just like it can in the rest of the body. Now, the two bacteria that are most commonly the culprit for infectious arthritis are Neisseria gonorrhoeae, gonorrhoeae, that's the first one. And then Staph aureus is the second one. And I've drawn this knee here because infectious arthritis usually happens or most commonly happens in the knee. And it's just one knee. It's not systemic, so there's no reason for both sides to be affected. It's usually just in one joint. And this joint that's affected here would be red and swollen and painful, just like having an infection anywhere in the body. This local inflammation will be there. Now, especially with Neisseria gonorrhoeae, you might be thinking, well, that sounds familiar. We talked about Reiter’s syndrome, which is one of the auto-immune arthritis illnesses. And you're right, gonorrhoeae is involved in Reiter's in the sense that after a local infection of gonorrhoeae, Reiter's syndrome can develop in the joints. But just to point out really quickly, in Reiter's, first of all the gonorrhoeae infection itself does not have to be local in the joint. It can be in the body in general. Reiter's. And also, we talked about in an auto-immune disease, the infection is in the blood, the orange dots being the bacteria. And then our body synthesizes these green, I think of like green soldiers fighting off the infection. And that's our immune system. And over time, after the original gonorrhoeae infection is gone, there's no more orange crystals in there, no more bacteria. But the immune system is still present. And the immune system is confused about what it's attacking. And it starts attacking this joint, which had nothing to do with gonorrhoeae in the first place. Now, this is a completely different process, and that's why it's called auto-immune, because our body's own immune system is causing the damage on our own tissue. But in infectious arthritis it's different because the gonorrhoeae is one locally in the knee. I'll draw the orange crystals here. It's not elsewhere in the body. And it's an active infection, meaning it's not been cleared. And the immune system is trying to get rid of it locally. That's why infectious arthritis is not an auto-immune disease. It's simply an infection in the body. So aside from the fact that it affects usually one joint at a time, usually the knee, also if we look at the onset of time, this being the time and this being the symptoms, it's gonna start very abruptly. So as you go along, suddenly the symptoms begin, as opposed to auto-immune diseases having a waxing and waning, up and down kind of course. The diagnosis of infectious arthritis should be pretty straightforward. When you're seeing a swollen, red knee like this, infection should always be one of the possibilities. And you do a fluid analysis. So you take a needle, go into the joint space, draw the fluid, and you can just test it for bacteria. If it's infectious arthritis, the bacteria will grow from the fluid. But also, a person having an infection anywhere is gonna have an increased white blood cell count. This is a sign that the immune system has responded. And whether you have an infection in your knee or in your finger or in your lungs or in your gut, it's gonna drive this up. So it's not specific, but it tells you that an infection is going on. Now, what we want to prevent in cases of all infections, including this one, is prevent it from becoming sepsis. And that's when the infection becomes systemic. It leaks into the blood. If you have septic arthritis, that's a step up in the severity. So if you have septic arthritis, this person would be very sick overall. They should have fevers and symptoms that are not limited to just one joint. And in this case when you do a blood culture, you should be able to grow bacteria from that. But honestly, we want to try to keep it from becoming septic arthritis because the treatment, the treatment for this is very simple, actually, just antibiotics. We know how to deal with infections. And that's why don't want to miss diagnosing this because the treatment is straightforward. Now, granted, infectious arthritis is not the only disease where we can have a red, swollen, and painful joint. But because of the fact that it can do a lot of permanent damage and it's easy to treat, we routinely take out the fluid from the knee just to analyze it. And the fluid content will tell us a lot. And by the way, just to be complete, Staph aureus is a bacteria that we have on our skin. All of us have it. And it's usually not harmful on the surface of our skin. But if it gets into a sterile area, it can cause infection. And Neisseria gonorrhoeae is associated with sexually transmitted infections. And that's why in terms of the people who get this, Staph aureus seems to be in older people or in older children, while gonorrhoeae is more likely in young adults. So infectious arthritis, not auto-immune, but does involve the immune system in the way that every infection in the body does. And the treatment is antibiotics.