Infectious diseases continue to play an important role in the current human society causing millions of deaths annually. Why infectious diseases still exists is unclear. The conventional wisdom suggests that with time, virulent pathogens should become harmless to ensure their persistence in the host population. New studies, however, have pointed out that virulent pathogens may still persist in the host population if pathogen''s virulence is linked to the pathogen transmissibility. Using mathematical modelling, this book investigates the potential important role of the host''s immune system in the evolution of pathogen virulence. The importance of various factors, such as the mechanism of pathogen virulence and immune status of the host, on the evolution of pathogens is considered. Models are also used to investigate details of the within-host dynamics of the T cell responses to viruses and to quantify rates of division and death of T cells in response to immunodepletion. This book will be useful for students and professionals interested in application of mathematical modeling for understanding of within-host dynamics and evolution of infectious diseases.