ABSTRACT
One of the most important objectives for infectious disease epidemiology is obviously to measure infectivity. Any numerical value given for infectivity will, however, have to be accompanied by a definition of the type of contact to which it applies. The sexual transmission of infections provides another field of study, where the type of contact is sufficiently standardized to allow for calculations of attack rate (AR). Part of the semantic problem comes from lack of reflection over the concept of 'risk of becoming infected'. Many models for the spread of sexually transmitted infections assume a constant AR per intercourse, and that the risk of transmission will increase with the number of intercourses that an infectious and a susceptible have. The occurrence of infection was established with both virological and serological criteria. Infectivity must be distinguished from 'risk of becoming infected', since this risk will also depend on prevalence of infectious sources in the environment.
