Reactive arthritis, formerly known as Reiter's syndrome, is a condition that causes inflammation (redness and swelling) in various places in the body.
It usually develops following an infection, and in most cases clears up in a few months without causing long-term problems.
The three most common places affected by reactive arthritis are:
However, most people will not experience all of these problems.
Read more about the symptoms of reactive arthritis.
For reasons that are still unclear, the immune system (the body's defence against infection) appears to malfunction in response to the infection and starts attacking healthy tissue, causing it to become inflamed.
Read more about the causes of reactive arthritis.
See your GP if you have swollen and painful joints, especially if you have recently had symptoms of an infection such as diarrhoea or pain when passing urine.
There is no single test for reactive arthritis, although blood and urine tests, genital swabs and X-rays may be used to check for infection and rule out other causes of your symptoms.
Your GP will also want to know about your recent medical history, such as whether you may have recently had a bowel infection or an STI.
There is currently no cure for reactive arthritis, but most people get better in around six months. Meanwhile, treatment can help to relieve symptoms such as pain and stiffness.
Severe symptoms may require more powerful steroid medication (corticosteroids) or disease-modifying anti-rheumatic drugs (DMARDs).
Read more about treating reactive arthritis.
Reactive arthritis can occur at any age, but it most commonly affects young adults aged 20-40.
Men are generally affected more than woman – particularly in cases linked to STIs, which are estimated to be about 10 times more common in men.
People who have a certain gene called HLA-B27, which is found in around one in every 10 people in the UK, are about thought to be around 50 times more likely to develop reactive arthritis than those who don't have this gene.
The most effective way to reduce your risk of reactive arthritis is to avoid the STIs and bowel infections that most commonly cause the condition.