Reiter's syndrome is a reaction to bacterial infection in other body systems. Although many people contract these infections, few of them go on to develop Reiter's syndrome. The mechanism that makes an infection progress to Reiter's syndrome is not well understood.

In the urogenital system, the initial infection may be caused by bacteria such as Chlamydia trachomatis or Ureaplasma urealyticum, which are transmitted through sexual contact. The initial infection may also be caused by the human immunodeficiency virus (HIV), the virus that causes acquired immune deficiency syndrome (AIDS). In the gastrointestinal system, some infections with certain microorganisms, including the Salmonella, Campylobacter or Shigella bacteria, may progress to Reiter's syndrome. These bacteria may enter the body when a person comes into contact with contaminated feces or eats or handles food contaminated with the bacteria, such as raw or undercooked meat.

Elements called risk factors may increase the likelihood of developing a disease or condition. However, not everyone with the disease may exhibit risk factors and not everyone with risk factors may develop the disease. Risk factors associated with Reiter's syndrome include:

• Sex and age. Reiter's syndrome is much more common in men than in women, and most common in men between the ages of 20 and 40. Most men with Reiter's syndrome develop it from chlamydia, ureaplasma or HIV/AIDS. Most women with Reiter's syndrome develop it from a gastrointestinal infection.
  
  
• Genetics. Studies show that a genetic marker called HLA-B27 may indicate a tendency to develop Reiter's syndrome. About 8 percent of healthy people have this gene, according to the National Institutes of Health. However, about 80 percent of people with Reiter's syndrome have the gene. Although the presence of HLA-B27 may indicate a tendency to develop the syndrome, it is not a certainty. Of the people with the gene who are exposed to a triggering infection, only 20 percent develop Reiter's syndrome.