Infective mechanisms probably underlie a wide range of inflammatory arthropathies. There appears to be a spectrum of mechanisms ranging from the frankly septic, through low-grade infection with very small numbers of microorganisms in the joint to arthritides in which no hard evidence for an infective cause exists. In the midst of the spectrum lie 'post-infective' and 'reactive' arthritides, characterized clinically, genetically and by epidemiological links with infection. Identification of bacterial components within joint material from such patients suggested that the causes of the arthritis had been found. It is now clear that many bacteria are present in inflamed joints; establishing their significance will be of crucial importance, but not easy.