This self-contained textbook provides a comprehensive guide to the mathematical theory of disease transmission models. General theory is presented alongside models for specific diseases and related phenomena, with exposition that is accessible to those who are comfortable with calculus, elementary differential equations and linear algebra. The reader will gain insight into the modeling of: cross-immunity between different disease strains such as influenza; synergistic interactions between multiple diseases such as HIV and tuberculosis; diseases transmitted by viral agents, bacteria and vectors; and, both epidemic and endemic disease occurrences. This text is ideal for a graduate level course for students in mathematics, or for students of epidemiology with strong mathematics backgrounds. The material within will also be of interest to researchers in epidemiology and related areas, and those involved with the development of public health policy.