As we enter the post-genomic era, there is an increasing need for accurate methods of identifying host and pathogen factors that contribute to bacterial, viral and fungal disease. In addition, there is a requirement for fast and precise techniques to evaluate potential therapies for the prevention of infectious diseases. The development of useful and cost-effective model systems will be crucial in advancing our knowledge of all aspects of microbial pathogenesis. In this series, we will learn of animal models used to investigate diseases caused by a wide variety of pathogens, including HIV, Vibrio cholerae and Psaudomonas aeruginosa. A description of a model system specifically designed to study innacellular pathogens will be presented, as,will a variety of the techniques currently used to exploit other useful models of infection. Additionally, a description of the mathematical models used to analyse the population biology of human onchocerciasis will be discussed. The series begins with an intriguing look at the possible connections between an endogenous retrovirus, the infectious agent of scrapie and accelerated senescence in a mouse model of early aging.