2. Septic Arthritis
Infants and older adults are susceptible to septic arthritis. If you have sudden onset of knee, hip, or shoulder pain and a fever after an infection or if you get a penetrating injury to the joint, see your doctor. The germs from the infection may have traveled to a joint and set up shop. The infection occurs quickly and damages cartilage and bone.
Risk factors include existing joint problems, certain medications, fragile skin, a weakened immune system and joint trauma.