American trypanosomiasis, or Chagas disease, is a parasitic disease caused by the haemoflagellate protozoa, Trypanosoma cruzi. The human infection occurs only in the Americas, where it is widely distributed in the periurban and rural areas of tropical and subtropical countries, from Mexico to Argentina and Chile. It is transmitted to man and other mammals mainly through insects, the triatomine bugs. As an enzootic disease, it extends from approximately latitude 42.5 degrees N (northern California and Maryland) to latitude 43.5 degrees S (southern Argentina and Chile). The results of several serological surveys indicate an overall prevalence of 16-18 million infected individuals. Up to 30% of those infected will develop the cardiac and/or hollow viscera irreversible lesions that characterize chronic Chagas disease. The endemic countries can be divided into four groups according to several indicators such as the number of confirmed human cases, the prevalence of seropositive tests in blood donors and population samples, the presence of infected vectors and reservoirs, and the existence or absence of coordinated actions towards the control of this disease. The domestic cycle of transmission, involving man and domestic animals such as dogs, cats, and domestic triatomine bugs, is the one that maintains the infections in the rural and periurban areas. Some triatomine species are well adapted to human dwellings where human and animal reservoirs are in intimate contact. The poor socioeconomic condition of the population and the domestic nature of the vector play crucial roles in maintaining the infection at an endemic level.(ABSTRACT TRUNCATED AT 250 WORDS)