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Reactive arthritis typically begins about 1 to 3 weeks after infection. The bacterium most often associated with reactive arthritis is Chlamydia trachomatis, commonly known as chlamydia (pronounced kla-MID-e-a). It is usually acquired through sexual contact. Some evidence also shows that respiratory infections with Chlamydia pneumoniae may trigger reactive arthritis.
Infections in the digestive tract that may trigger reactive arthritis include Salmonella, Shigella, Yersinia, and Campylobacter. People may become infected with these bacteria after eating or handling improperly prepared food, such as meats that are not stored at the proper temperature.
Doctors do not know exactly why some people exposed to these bacteria develop reactive arthritis and others do not, but they have identified a genetic factor, human leukocyte antigen (HLA) B27, that increases a person’s chance of developing reactive arthritis. Approximately 80 percent of people with reactive arthritis test positive for HLA-B27. However, inheriting the HLA-B27 gene does not necessarily mean you will get reactive arthritis. Eight percent of healthy people have the HLA-B27 gene, and only about one-fifth of them will develop reactive arthritis if they contract the triggering infections.