An overview of epidemiology of R. equi infection in foals is presented, emphasizing the importance of the virulence-associated antigens and plasmids as epidemiological markers. The monoclonal antibody-based colony blot test has been developed to identify rapidly and accurately virulent R. equi. Epidemiological studies conducted during the recent 5 years have revealed that: (1) avirulent R. equi are widespread in the feces of horses and their environment on every farm; (2) the feces of horses and the environment of the horse farms having endemic R. equi infections demonstrated heavy contamination with virulent R. equi, but the farms without the problem did not, thus suggesting that foals bred on a farm with endemic disease are exposed more frequently to virulent R. equi in their environment than those of a farm without the problem; (3) only virulent R. equi are isolated from lesions of naturally infected foals, showing that natural infections in foals are principally by virulent R. equi, but not avirulent organisms; (4) infected foals which constantly shed large quantities of virulent R. equi in their feces are the major source of virulent R. equi, which this may be the mechanism of progressive development of infection on farms with a history of the disease. At present, farms with a potential for endemic infection can be distinguished on the basis of the contamination with virulent R. equi, so regular examination of foals and their environment by virulence markers might be the most practical approach to control R. equi infection on endemic farms.