Knee infections are most often caused by bacteria entering the joint, either through the bloodstream, a wound, or a medical procedure. The bacterium Staphylococcus aureus (staph) is responsible for the majority of cases in both adults and children. Left untreated, a knee infection can cause permanent cartilage destruction and joint damage within 24 to 48 hours, making it a medical emergency.
How Bacteria Reach the Knee Joint
Bacteria get into the knee through three main routes. The most common is through the bloodstream. An infection somewhere else in your body, such as a skin wound, urinary tract infection, or respiratory infection, can send bacteria circulating through your blood. The knee joint has a rich blood supply and no protective lining to filter out pathogens, so bacteria can settle there easily and multiply in the nutrient-rich joint fluid.
The second route is direct entry through a break in the skin. Puncture wounds, deep cuts near the knee, animal or human bites, and open fractures all create a path for bacteria to reach the joint. About 5% of joint infections involve multiple types of bacteria at once, and these polymicrobial infections typically result from trauma or abdominal infections that introduce several organisms simultaneously.
The third route is through medical procedures. Any time a needle or surgical instrument enters the knee, there is a small window for bacteria to follow. This includes joint injections, arthroscopic surgery, and knee replacement.
Knee Injections and Surgical Procedures
Corticosteroid and hyaluronic acid injections for knee osteoarthritis carry a low but real risk of introducing infection. The estimated rate is roughly 10 to 40 infections per 100,000 injections. Most of these cases trace back to lapses in sterile technique rather than inherent danger from the procedure itself. A CDC investigation of a septic arthritis outbreak in New Jersey found that the physician involved was skipping hand hygiene, using nonsterile gloves, and swapping multiple syringes on a single needle left sitting in the joint space.
Knee replacement surgery carries an infection rate of about 2% or less. These prosthetic joint infections can be especially difficult to treat because bacteria form protective layers on the surface of the artificial joint. When standard cultures come back negative but infection is still suspected, fungi and mycobacteria account for over 85% of those hard-to-detect cases.
Who Is Most at Risk
Certain health conditions make you significantly more vulnerable to a knee infection. Rheumatoid arthritis is one of the strongest risk factors because it damages the joint lining and because the medications used to treat it suppress the immune system. Diabetes impairs your body’s ability to fight infection and slows wound healing, making it easier for bacteria to gain a foothold. Other conditions that weaken the immune system, including HIV, cancer treatment, organ transplant medications, and chronic liver or kidney disease, also raise your risk.
People who already have joint damage from osteoarthritis or previous knee surgery are more susceptible because the compromised tissue is easier for bacteria to colonize. Intravenous drug use is another well-known risk factor, as repeated needle use can introduce bacteria directly into the bloodstream. Age plays a role too: very young children and adults over 65 face higher rates of septic arthritis.
Less Common Causes
While staph bacteria dominate the statistics, other organisms can infect the knee. Streptococcus species are the second most common bacterial cause. In sexually active young adults, gonorrhea is a notable cause of joint infection that often presents differently, with milder symptoms and involvement of multiple joints.
Fungal infections and mycobacteria (the family of organisms that includes tuberculosis) are rare but important causes, particularly in people with weakened immune systems or those with artificial joints. A systematic review found at least 17 different mycobacterial species responsible for prosthetic knee and hip infections. Tuberculosis accounted for about 43% of those cases, while faster-growing species made up the rest. These atypical infections tend to develop slowly and are easy to miss on standard lab tests.
How It Differs From Gout and Other Conditions
A hot, swollen, painful knee isn’t always infected. Gout and a related condition called pseudogout can look nearly identical, with sudden onset of severe pain, redness, and swelling. The key difference lies in the joint fluid. In a true infection, the fluid typically contains more than 50,000 white blood cells per cubic millimeter, with over 75% being a type called neutrophils. Counts above 100,000 are even more strongly linked to infection. In gout, the white blood cell count usually stays below 50,000, and the fluid contains characteristic urate crystals that confirm the diagnosis.
On imaging, abscesses in the soft tissue around the knee appear only in infection, not gout. The pattern of tissue thickening inside the joint also differs: infections tend to produce a layered appearance on MRI, while gout shows a more uniform, smooth pattern. Still, distinguishing these conditions reliably requires drawing fluid from the joint, as symptoms alone overlap too much to tell them apart.
How Quickly Damage Occurs
Speed matters enormously with a knee infection. Permanent bone loss beneath the cartilage, destruction of the cartilage itself, and lasting joint dysfunction can begin if appropriate treatment isn’t started within 24 to 48 hours of symptom onset. The enzymes released by bacteria and your own immune response both eat away at cartilage, and unlike bone, cartilage has almost no ability to regenerate. Staph infections in particular carry a mortality rate that can exceed 50% in severe cases, underscoring how aggressively these infections need to be managed.
The classic warning signs are a knee that becomes rapidly swollen, warm to the touch, intensely painful, and difficult to bend or straighten, often accompanied by fever and chills. If the infection follows surgery or an injection, symptoms may develop days to weeks after the procedure rather than immediately.