The killer phenotype of Saccharomyces cerevisiae strains is based upon the presence of viruses in the cytoplasm of the yeast cells. Resent analysis of the molecular basis in these phenomenon let researchers to reveal its molecular mechanism and ecological function. Mammalian transmissible spongiform encephalopathiesare is likely due to the propagation of an abnormal form of some protein. Such infectious agents, which are termed prions, exist in yeasts. This review highlights the variety of infectious elements present in Saccharomyces cerevisiae as well as their influence on the yeasts properties.