Arthritis is inflammation inside a joint that causes pain, stiffness, and sometimes swelling

Arthritis happens when the tissues inside a joint become inflamed. The inflammation can damage the cartilage—the smooth material that covers the ends of bones—and the bone itself. Over time, this damage makes the joint hurt, feel stiff, and sometimes swell. The pain is usually worse after you use the joint or when you first wake up.

Arthritis is not a single disease. There are over 100 types, but osteoarthritis and rheumatoid arthritis account for most cases. They start differently, progress differently, and respond to different treatments. Understanding which type you have matters because the treatment that helps one type may not help the other.

Key Takeaways

  • Osteoarthritis develops when cartilage wears down over years of use, usually starting in your 50s or later, while rheumatoid arthritis is an autoimmune disease that can start at any age.
  • Osteoarthritis typically affects one or a few joints, while rheumatoid arthritis usually affects multiple joints on both sides of the body at the same time.
  • Both types cause pain and stiffness, but rheumatoid arthritis also causes systemic symptoms like fatigue and fever because it is a whole-body immune condition.
  • X-rays and blood tests can help distinguish between types, and early diagnosis matters because treatments for rheumatoid arthritis work better when started sooner.

Osteoarthritis: wear-and-tear damage to cartilage

Osteoarthritis develops when the cartilage that cushions the ends of your bones gradually wears away. This usually happens over many years. The cartilage becomes thinner and rougher, and the bone underneath can start to break down. As the joint surface becomes uneven, movement becomes painful and the joint may feel stiff, especially in the morning or after sitting still.

Osteoarthritis most often affects the knees, hips, hands, and spine—the joints that bear weight or get used repeatedly. It is more common as you age because cartilage damage accumulates over decades. Injury to a joint, being overweight, or doing repetitive motions can speed up the process. Unlike rheumatoid arthritis, osteoarthritis does not cause fever or affect other parts of your body.

Rheumatoid arthritis: an immune system attack on joints

Rheumatoid arthritis happens when your immune system mistakenly attacks the lining of your joints. This lining, called the synovium, becomes inflamed and swollen. Over time, this inflammation can damage the cartilage and bone inside the joint. Unlike osteoarthritis, rheumatoid arthritis can start at any age, including in young adults, and it often affects multiple joints at the same time—usually the same joints on both sides of your body.

Because rheumatoid arthritis is a systemic disease, it affects your whole body, not just your joints. You may feel fatigued, run a low fever, or lose your appetite. Some people develop rheumatoid nodules—firm bumps under the skin—or eye inflammation. Blood tests can detect specific markers, like rheumatoid factor or anti-CCP antibodies, that help confirm the diagnosis. Early treatment can slow or even stop the progression, which is why getting diagnosed quickly matters.

Other types of arthritis

Beyond osteoarthritis and rheumatoid arthritis, there are many other forms. Psoriatic arthritis develops in some people with the skin condition psoriasis. Gout occurs when uric acid crystals build up in a joint, usually the big toe, causing sudden severe pain. Lupus and other autoimmune diseases can cause arthritis as one of their symptoms. Infectious arthritis happens when bacteria or viruses enter a joint.

Each type has different causes and different treatments. Some are temporary—infectious arthritis may resolve once the infection is treated. Others are chronic and require long-term management. A doctor can run tests to narrow down which type you have based on your symptoms, the pattern of affected joints, blood work, and imaging.

How doctors tell the difference

Your doctor will start by asking when the pain began, which joints hurt, whether the pain is worse at certain times of day, and whether you have other symptoms like fever or fatigue. They will examine your joints for swelling, warmth, and range of motion. They may order an X-ray to look for cartilage loss or bone damage, or an ultrasound to see inflammation in the joint lining.

Blood tests can reveal clues about what type of arthritis you have. Rheumatoid arthritis often shows up as rheumatoid factor or anti-CCP antibodies in the blood. Gout can be confirmed by finding uric acid crystals in fluid drawn from the joint. Osteoarthritis usually does not show specific markers in blood tests, so diagnosis relies more on symptoms, imaging, and ruling out other types.

What happens inside an inflamed joint

When a joint becomes inflamed, the synovium swells and produces extra fluid. This fluid buildup causes the joint to feel puffy and can limit how far you can move it. The inflammation also triggers pain signals. Over time, inflammatory chemicals can damage the cartilage and bone, making the damage permanent if the inflammation is not controlled.

In osteoarthritis, the damage is usually mechanical—cartilage simply wears away from friction and use. In rheumatoid arthritis, the damage is active and ongoing because the immune system keeps attacking. This is why treatments for rheumatoid arthritis focus on calming the immune response, while treatments for osteoarthritis focus more on reducing pain and keeping the joint moving.

Why early diagnosis matters

If you have rheumatoid arthritis, starting treatment within the first few months can prevent or slow permanent joint damage. Medications called disease-modifying antirheumatic drugs, or DMARDs, work best when started early. Waiting months or years before treatment allows more damage to accumulate, and that damage cannot always be reversed.

For osteoarthritis, early diagnosis helps you understand what is happening and start strategies to slow progression—like staying active, managing weight, and protecting the joint from further injury. Neither type of arthritis goes away on its own, but both can be managed to reduce pain and preserve function.

Frequently Asked Questions

Is arthritis the same as joint pain?

No. Joint pain can come from many causes—injury, overuse, infection, or inflammation. Arthritis is specifically inflammation inside the joint that causes ongoing pain, stiffness, and sometimes swelling. If your joint pain lasts more than a few weeks or keeps coming back, it may be arthritis.

Can you get arthritis in your 30s or 40s?

Yes. Osteoarthritis is rare before age 50, but rheumatoid arthritis, psoriatic arthritis, and gout can start at any age. If you have joint pain and swelling that does not go away after a few weeks, see a doctor for testing rather than assuming it will pass.

Does arthritis always show up on an X-ray?

Not always. Early osteoarthritis may not be visible on X-rays yet. Rheumatoid arthritis can cause joint damage that shows on X-rays, but in the early stages, blood tests and ultrasound may detect inflammation before X-rays do. Your doctor may use multiple tests to confirm a diagnosis.

Is arthritis hereditary?

Genetics play a role in both osteoarthritis and rheumatoid arthritis, but having a family member with arthritis does not mean you will definitely develop it. Environmental factors—injury, weight, activity level, and immune system triggers—also matter. Your risk is higher if a close relative has arthritis, but it is not may provide.