This graduate-level textbook appeals to readers interested in the mathematical theory of disease transmission models. It is self-contained and accessible to readers who are comfortable with calculus, elementary differential equations, and linear algebra. The book provides insight into modeling:* Cross-immunity between different disease strains (such as influenza) and the synergistic interactions between multiple diseases (e.g., HIV and tuberculosis).* Diseases transmitted by viral agents, bacteria, and vectors (e.g., mosquitos transmitting malaria to humans).* Both epidemic and endemic disease occurrences.