What is Reactive Arthritis?
Reactive arthritis is an uncommon, but potentially debilitating group of symptoms that can follow Salmonella, Campylobacter, and other foodborne illnesses.
Reactive Arthritis refers to a group of arthritic diseases that includes a subset formally known as “Reiter’s Syndrome” The old term Reiter’s syndrome has fallen into disfavor. In recent medical literature Reiter’s Syndrome is simply referred to as Reactive Arthritis which may or may not be accompanied by extraintestinal manifestations.
Reactive Arthritis is the name used to describe an uncommon, but potentially debilitating group of symptoms that follows a gastrointestinal, genitourinary, or viral infection. The most common gastrointestinal bacteria involved are Salmonella, Campylobacter, Yersinia, Shigella, E. coli, and Vibrio. The most common genitourinary causes are sexually transmitted diseases such as Chlamydia and Gonorrhea. The most common viral causes are the common flu, HIV, and Parvovirus.
The specific triad of arthritis, conjunctivitis, and urethritis was known as Reiter’s Syndrome. In many patients, however, only one or two of these symptoms many be present, such as arthritis and urethritis or arthritis and conjunctivitis.  A reactive arthritis may develop after a person eats food that has been tainted with the pathogenic bacteria. Many patients will get severe bloating, abdominal pain, and watery diarrhea. However in some patients, the initial infection may be milder and not easily recognized, but the reactive arthritis may still occur. Reactive arthritis typically involves inflammation of one joint (monoarthritis) or several joints (oligoarthritis), preferentially affecting those of the lower extremities. The most common joints affected are the knees and ankles. The pattern of joint involvement is usually asymmetric. Inflammation can also be commonly seen at an enthesis (a place where ligaments and tendons attach to bone), especially the front of the knee or the back of the ankle where the Achilles attaches to the calcaneal bone. This is referred to as an enthesopathy. It causes local swelling and pain with walking or exercise.
Salmonella has been the most frequently studied bacterium associated with reactive arthritis. Overall, studies have found rates of Salmonella-associated reactive arthritis to vary between 6% and 30%.  The frequency of post-infectious Reiter’s syndrome specifically, however, has not been well described. In a Washington State study of an outbreak of foodborne Salmonella gastroenteritis, 29% of patients developed arthritis, but only 3% developed the triad of symptoms associated with Reiter’s syndrome.  In addition, individuals of Caucasian descent may be more likely than those of Asian descent to develop reactive arthritis,  and children may be less susceptible than adults to reactive arthritis following infection with Salmonella. 
The frequency of acute reactive arthritis from other bacteria varies widely. The occurrence of new joint pain alone after enteric infection is reported to be between 1 and 4% in adults with Campylobacter or Shigella infections.  In another study, it has been reported to occur in 0.6% to 24% of Campylobacter gastroenteritis patients.  After Shigella infection the percentage of patients who exhibit subsequent reactive arthritis ranges from 1.5% to 7%. 
A clear association has been made between reactive arthritis and a genetic marker called the Human Leukocyte Antigen (HLA) B27 genotype. HLA is a major histocompatibility complex in humans; these are proteins present on the surface of all body cells that contain a nucleus, and are in especially high concentrations in white blood cells (leukocytes). It is thought that HLA-B27 may affect the elimination of the infecting bacteria or an individual’s immune response.  HLA-B27 has been shown to be a predisposing factor in one-half to over two-thirds of individuals with reactive arthritis. [8,1] While HLA-B27 does not appear to predispose to the initial infection itself, it increases the risk of developing arthritis that is more likely to be severe and prolonged. This risk may be slightly greater for Salmonella and Yersinia-associated arthritis than with Campylobacter, but more research is required to clarify this. 
Why Have Some Forms of Reactive Arthritis Been Called Reiter’s Syndrome?
One could also ask, “Why isn’t Reiter’s syndrome called Reiter’s syndrome anymore?” The answer is that the syndrome, or group of symptoms, is named for Hans Reiter who described a soldier with the triad of urethritis (burning and pain with urination), conjunctivitis (redness and pain of the eye), and arthritis (swelling and pain of the joints) after having bloody diarrhea in 1916. However, the history of this constellation of signs actually predates his description. And what is disturbing is that Reiter was a high-ranking Nazi official who was responsible for medical experiments in concentration camps. [9, 10] As a result, the term Reiter’s syndrome has fallen out of favor and Reactive Arthritis is preferred to describe the post-infectious arthritis which may be accompanied by extra-articular manifestations (such as urethritis and conjunctivitis).