When you think of arthritis, what probably comes to mind is osteoarthritis, the wear-and-tear disease that affects more than 30 million Americans.

What you may not realize: Other types of arthritis can ravage joints, including arthritis caused by infection. For example, a recent study published in Arthritis Care & Research found that more than 16,000 people go to the emergency room every year with septic arthritis, an infection that can cause irreversible damage and deformity to joints.

Infections play a role in many cases of arthritis, both acute and chronic. Because these types of arthritis are less common than osteoarthritis, they are often misdiagnosed or overlooked. What you need to know about…

Septic arthritis. Triggered by bacteria in the bloodstream that settle in a joint, this is the most serious form of infectious arthritis. Without prompt treatment, deep bone infections may occur and take months to resolve. One-third of people with septic arthritis suffer joint damage—and 10% die.

About half the time, the infection is in the knee, and pain and swelling are so severe that walking is difficult. The joint will also be red and hot. And you’ll have a fever and chills and feel very sick. The knee is a common site for septic arthritis because of its large size and location.

People who have weakened immune systems, including adults over age 65 and children, are the most common victims of septic arthritis. You’re also at higher risk if you already have a joint problem, such as osteoarthritis, gout or rheumatoid arthritis, which is an autoimmune disease…if you’re taking medications for rheumatoid arthritis, which suppress the immune system…if you have an immune-weakening disease, such as diabetes or cancer…or if you have fragile skin that is easily injured and heals poorly (a fairly common problem among older adults and those with diabetes), allowing bacteria ready access into your bloodstream.

Many types of bacteria can cause septic arthritis. The most common bacterial culprits are Staph and Streptococcal species. If the infection isn’t stopped, bacteria can destroy cartilage, causing permanent damage.

What to watch out for: Sudden, severe pain in a knee or other joint, and flulike symptoms, including fever and chills.

Treatment: Go to the emergency room or see a doctor—immediately. The joint will be drained with a needle or tube (arthroscopy), and the fluid will be cultured to identify the bacteria. It’s likely you’ll also get blood tests to help pinpoint the infection and X-rays to see if the joint has been damaged.

If you’re diagnosed with septic arthritis, you’ll receive IV antibiotics, followed by oral antibiotics. The usual antibiotic treatment duration is about six weeks. If antibiotics aren’t effective, you may need surgery to drain the infection.

Reactive arthritis. This type of arthritis can plague joints for weeks to years. Doctors aren’t certain if reactive arthritis is an infection of the joint…or a joint-centered inflammation triggered by an infection elsewhere in the body. Either way, the arthritis is typically caused by either a sexually transmitted bacterial infection, such as chlamydia or gonorrhea, or a gastrointestinal infection, such as C. difficile or salmonella. Food poisoning is a common trigger.

What to watch out for: Joint pain that develops a few weeks or months after a sexually transmitted or gastrointestinal infection.

Treatment: A blood test will be given to detect the bacteria. If it’s positive, you’ll take antibiotics that target the organism. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Motrin)…corticosteroids…or antirheumatic drugs, such as methotrexate (Trexall), may also be prescribed.

Lyme arthritis. Lyme disease, a bacterial infection from a tick bite, is found throughout the US, but mainly in the Northeast, from Maine to Virginia, and in Minnesota, Wisconsin, Michigan and northern California. Some people never overcome the infection and develop chronic Lyme disease. Among those, more than half develop Lyme arthritis—one or more swollen joints (usually a knee), with pain (typically mild) that is intermittent or constant.

What to watch out for: Lyme arthritis usually develops several months after the tick bite. As with all types of arthritis, joint swelling and pain can occur.

Treatment: Lyme arthritis often resembles reactive arthritis. To make a definitive diagnosis, your doctor will order a blood test to detect antibodies to B. burgdorferi, the bacterium transmitted from the tick bite. The doctor may also remove fluid from your joint for a polymerase chain reaction (PCR) test, which detects the presence of DNA from B. burgdorferi.

If one or both of these tests are positive, your doctor will probably prescribe oral or intravenous antibiotics for one to three months. In most cases, this treatment cures Lyme arthritis, especially if it’s initiated early on.

However, in some patients, the treatment fails, and chronic arthritis, as well as other symptoms (such as fatigue, headache and difficulty concentrating), may persist. If the disease isn’t controlled, a drug used for rheumatoid arthritis—a disease modifying antirheumatic drug, or DMARD—often helps control the Lyme arthritis. Some patients require long-term treatment, while others improve over a period of months.

Viral joint infections. Many viruses can trigger acute arthritis, but the joint pain that results is usually mild and goes away on its own after a few weeks. Viral infections that can cause arthritis include…

Zika virus, from mosquitoes that carry it.

Epstein-Barr, the virus that causes mononucleosis.

Hepatitis A and B, the liver-infecting viruses that cause about one-third of virus-triggered arthritis.

Parvovirus, a respiratory infection common in adults who are routinely exposed to children, the primary carriers of this infection, which causes a distinctive face rash.

What to watch out for: Sudden, mild joint pain (viral arthritis can affect almost any joint).

Treatment: Your physician will order a blood test for antibodies to specific viruses that can cause acute arthritis. Pain control is the goal, typically with an over-the-counter NSAID, such as ibuprofen or naproxen (Aleve).

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