We introduce an extension of the SIS epidemic model that describes infection, mutation and curing for a whole hierarchy of viruses, resembling a nested spreading process. In our model, high level viruses are only allowed to spread to nodes that have acquired a lower level of infection before. The simplest case of two viruses, in which one "superinfects" the other, shows already rich dynamics that are difficult to predict by common mean-field approximation techniques in certain cases. We derive an exact Markovian description for superinfection in the complete network and the star network showing that the steady state of the epidemic process is highly sensitive to the spreading rate of both viruses. Taking the spreading rates into account, we outline conditions for epidemic outbreaks, coexistence of both viruses and extinction cycles.