Infectious arthritis is un infection in the fluid (synovial fluid) and tissues of a joint.

Infecting organisms, mainly bacteria, usually reach the joint through the bloodstream, but a joint can be infected directly if it's contaminated by surgery, an injection, or an injury. Different bacteria can infect a joint, but the bacteria most likely to cause infection depend on a person's age. Staphylococci, Hemophilus influenzae, and bacteria known as gram-negative bacilli most often infect babies and young children, whereas gonococci (bacteria that cause gonorrhea), staphylococci, and streptococci most often infect older children and adults. Viruses--such as the human immunodeficiency virus (HIV), parvoviruses, and those that cause rubella, mumps, and hepatitis B--can infect joints in people of any age. Chronic joint infections are most often caused by tuberculosis or fungal infections.

Symptoms

Infants usually have fever and pain and tend to be fussy. Cenerally, infants don't move the infected joint because moving or touching it is painful. In older children and adults who have bacterial or viral joint infections, symptoms usually begin very suddenly. The joint usually becomes red and warm, and moving or touching it is very painful. Fluid collects in the infected joint, causing it to swell and stiffen. Symptoms also include fever and chills.

The joints most commonly infected are the knee, shoulder, wrist, hip, finger, and elbow. Fungi or mycobacteria (bacteria that cause tuberculosis and similar infections) usually cause less dramatic symptoms. Most bacterial, fungal, and mycobacterial infections affect only one joint or, occasionally, several joints. For example, the bacteria that cause Lyme disease most often infect knee joints. Conococcal bacteria and viruses can infect many joints at the same time.

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