Abstract
The dynamics of an infectious disease is usually approached at the population scale. However, the event of an epidemic outbreak depends on the existence of an active infection at the level of the individual. A full study of the interaction between the infection of hosts and its transmission in the population requires the incorporation of many factors such as physiological age, age of infection, risk conditions, contact structures and other variables involving different spatial and temporal scales. Nevertheless, simple models can still give some insight on the intricate mechanisms of interactions necessary for the occurrence of an epidemic outbreak, in particular, one can explore the role that the reproductive numbers at the between-host and within-host levels play. In this talk I will review some results on the epidemiology of between-host, within host interactions.