Several different kinds of infections can bring about arthritis, including bacteria, viruses, and fungal infection. Bacterial knee arthritis can cause something called a septic joint. This is usually easy to diagnose because the joint becomes suddenly painful, with the classic signs of infection, including redness, swelling, tenderness, and difficulty moving the joint. Typically, you also develop a fever, which can be very high. The knee is the most common area infected, but other joints that can get infected are the hip, shoulder, wrist, and ankle. Because it’s very unusual for it to affect your fingers, it is relatively easy to differentiate bacterial arthritis from rheumatoid arthritis. Moreover, there is usually fluid inside the joint, which your doctor can drain with a needle and send to a lab to identify the bacteria. While these cultures are positive in only one-third of the cases, the blood can be cultured too, and this tests positive in up to 60 percent of adults.

For example, a joint can develop a staph infection after an injury allows this bacterium into your body through the skin. Another test looks for the bacteria DNA in the joint fluid. The most common bugs are the Yersinia species, Chlamydia species, Mycoplasma hominis, Ureaplasma species, Borrelia burgdorferi (Lyme), and Neisseria gonorrhoeae. And sometimes, if an infection such as Mycoplasma goes undetected and untreated, it can become a trigger for RA. The most important thing to keep in mind is that if you experience sudden pain in a large joint such as your knees or hips, accompanied by a fever, see your doctor because you might have an infected joint that needs treatment with antibiotics.

A diagnosis is made because they have a high concentration of a distinct antibody called anti-U1-ribonucleoprotein (RNP). Most people with MCTD will end up with one of the three overlapping conditions years later. A confirmed diagnosis of MCTD requires a positive blood test, but you also need to have the clinical picture of joint pain, muscle pain, and fatigue. With septic arthritis, the bacteria enter the joint after traveling through the blood, which is why the joint infection usually comes after an infection begins somewhere else.