It's happened to everyone. Spend too much time on the tennis court or jogging and you feel the pain a few hours later. Maybe your knees tighten, or an elbow swells. If you're lucky, you'll pop a couple of pain relievers and feel fine. At worst, you'll have to forgo a day of exercise to reflect that you're not a kid anymore.

But some people are not so lucky. They'll suffer with pain for weeks, sometimes years, before discovering that what they've got isn't a sprained back or sore muscles. It's arthritis, a condition the Centers for Disease Control (CDC) calls one of the most prevalent in the United States. Right now, arthritis affects 43 million Americans. The CDC estimates that in two more decades it will affect as many as 60 million.

Arthritis and its nearly 200 related conditions (lupus, carpal tunnel syndrome, Lyme disease and gout are all associated with this disease) erode the cartilage that cushions bones at the joints. As the cartilage disintegrates, it can break off into the joint and irritate muscles and other tissues. Eventually, unprotected bones may begin to grind together during movement, causing excruciating pain. The CDC says rheumatic conditions are the leading cause of disability in people 15 years or older, and they cost $65 billion in health care every year.

Of the two most common forms-osteoarthritis and rheumatoid arthritis-osteoarthritis is the most common. It often comes from a joint injury or a genetic predisposition for joints to wear out prematurely. Although osteoarthritis can affect the hands and back, the Mayo Clinic says it accounts for 70 percent of all knee and 60 percent of all hip replacements. Risk factors include heredity, excess weight and injury.

Much more serious, according to Dr. Richard Yonker of the Arthritis Center in Sarasota, is rheumatoid arthritis, since it is a disease of the autoimmune process. Its hallmarks are redness, swelling and inflammation that gradually worsen and can lead to permanent disability. It affects all ages, especially those between the ages of 25 and 50.

For people suffering the severest forms of this disease, climbing a flight of stairs requires Herculean effort. Hip bones grind together. Fingers that once danced across piano keys or threaded fishing lures curl inward toward the palm like shrunken claws. Opening a jar of peanut butter is nearly impossible. Even worse, rheumatoid arthritis does not limit itself to joint destruction. It also attacks the eyes and internal organs such as the heart and lungs.

Doctors classify the disease as rheumatoid rather than osteoarthritis when patients complain of three or more painful joints at the same time along with fatigue, fever and weight loss. Also, rheumatoid arthritis usually affects the wrist and all the hand joints except those closest to the fingernails.

Researchers are still trying to unravel the mystery of why women make up 60 percent of arthritis cases (or why women who drink at least four cups of decaffeinated coffee a day appear to be twice as likely to develop rheumatoid arthritis). Some researchers suspect that a virus causes the disease, but they aren't sure what that virus is.

What they do know is that rheumatoid arthritis is not confined to old age. The Arthritis Foundation says that although it affects one of every two people over 65, nearly three out of five are younger-and juvenile rheumatoid arthritis is one of the most common chronic childhood diseases.

Unfortunately, an estimated 200,000 people fail to seek treatment because they attribute their aches and pains to aging. Yonker says that not only can arthritis be treated, but some forms of osteoarthritis can even be prevented with weight control and efforts to avoid occupational and sports-related injuries.

The Arthritis Foundation's Internet Web site (www.arthritis.org) has a comprehensive list of what it believes are the most recent important developments in this area.

Part of that news involves medication. Acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) are the first line of defense when treating osteoarthritis, but each has its drawbacks. Acetaminophen relieves pain, but doesn't reduce inflammation. NSAIDs reduce swelling and pain but can cause stomach ulcers, especially in women. Vioxx, one of a widely advertised class of drugs called COX-2 inhibitors, provides alternative treatment, but came under fire for serious cardiac problems after the FDA said it carries twice the risk of heart attacks as naproxen, a common over-the-counter NSAID. They even recommended that it not be taken by arthritis patients with a history of heart disease.

Yonker says that Vioxx doesn't cause heart attacks, it just doesn't seem to prevent them with the same success of older NAISDs. "That's because older drugs thin the blood," he says. "The new (COX-2 inhibitors) do not." That means that people who are predisposed to blood clots that may eventually lead to cardiac disease may benefit more from an NAISD, even with its associated stomach ills. Yonker still has confidence in Vioxx but is careful not to prescribe it to patients with heart disease or hypertension.

People with rheumatoid arthritis face a different set of challenges. They, too, can use NSAIDs, but drugs that prevent the immune system from attacking joints cause significant side effects. Steroids reduce inflammation and swelling, but increase the risk of osteoporosis if taken for too long. Other drugs adversely affect blood counts and liver enzymes and are toxic to bone marrow. Yonker and his colleagues at the Arthritis Center are testing several new drugs (Embrel, Remicade and Kineret) that he says cause less joint destruction and don't produce as many side effects.

One treatment getting attention isn't even a drug. When herbalists and natural food stores began touting glucosamine as an effective agent against arthritis, the medical community was skeptical. Now a European study published in the Lancet Medical Journal shows that glucosamine can halt loss of joint cartilage in people with mild to moderate osteoarthritis of the knee. The study reported that people taking this supplement enjoyed the same pain relief as with NSAIDs, and the Arthritis Foundation says some research indicates glucosamine may even slow cartilage damage. Yonker uses glucosamine along with NAISDs. "Non-steroidal drugs typically don't stop joint destruction," he says, "but in osteoarthritis, glucosamine might." The National Institutes of Health is now conducting its own clinical study on this supplement.

Women's lifestyle issues are also making news. A University of Alabama study found that women who smoke are twice as likely to develop rheumatoid arthritis as those who do not. Furthermore, smokers without a family history of rheumatoid arthritis had higher rates of the condition than non-smokers with a family history of the disease.

Women also suffer from osteoarthritis of the knee twice as often as men, and researchers at Harvard University think their shoes may be responsible. They say that since any type of heel alters the ankle's normal stabilization function, compensations by the knee and hip joints can lead to their degeneration. Their study indicates that wide heels cause just as much damage as high heels.

On the alternative medicine front, acupuncture is a growing trend in arthritis treatment. "We actually have one doctor who practices acupuncture here [at the Arthritis Center]," says Yonker. A 1997 National Institutes of Health panel conceded that acupuncture is an effective addition to conventional osteoarthritis treatments. The panel even encouraged insurance companies to cover the procedure. The panel was less enthusiastic about acupuncture for rheumatoid arthritis, but Yonker says it seems to work in some cases for back pain and fybromyalgia. It at least provides temporary relief from the pain; Yonker says there is no cure.

He says the trick is to treat rheumatoid arthritis before it does irreparable damage. The disease destroys joints early, and it's difficult to diagnose. "There's no one test," he says. "It requires a medical history, physical examination, blood tests and X-rays." That's why it's so important to see a trained rheumatologist early. "Once you damage a joint, it's not reversible." But, he stresses, by seeking aggressive treatment, "You can live a normal life with rheumatic disease."