Fuzzy theory techniques are applied to susceptible-infectious SI epidemiological models. The transmission coefficient is considered as a fuzzy set and the mean number of infected individuals is compared with the trajectory of the mean virus charge I((v) over bar, t). Also, the basic reproduction value R-0(f) is discussed for this formulation and, using R-0(f), a control policy of the disease is discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.454195316191628