Infectious arthritis is joint pain, soreness, stiffness and swelling caused by a bacterial, viral, or fungal infection that spreads from another part of the body. Depending on the type of infection, one or more joints may be affected. Certain bacteria can cause a form of Infectious arthritis called reactive arthritis, which appears to be caused by the immune system reacting to bacteria, rather than by the infection itself. In reactive arthritis, joint inflammation develops weeks, months or even years after the infection. Reactive arthritis happens most commonly after infections of the genital and gastrointestinal tracts. To diagnose Infectious arthritis, your health care provider may do tests of your blood, urine, and joint fluid.