This review article explores the population biology of infectious diseases, focusing on how infectious diseases regulate host populations. It discusses the dynamics of microparasitic and macroparasitic infections, emphasizing the role of transmission parameters, immune responses, and population regulation. The article presents mathematical models that explain how infectious diseases can influence the growth rates of host populations, using laboratory experiments on mice as a case study. It highlights the importance of infectious diseases in regulating wild mammal and bird populations, and discusses the evolutionary relationships among transmission parameters. The article also addresses the implications of indirect transmission and the broader ecological and evolutionary significance of infectious diseases. The first part of the article introduces the mathematical models used to analyze infectious diseases, while the second part, to be published separately, extends these models to include indirectly transmitted infections and their general implications. The article concludes by emphasizing the role of immune responses in determining the population consequences of infectious diseases and the importance of understanding these dynamics for predicting and managing disease outbreaks.This review article explores the population biology of infectious diseases, focusing on how infectious diseases regulate host populations. It discusses the dynamics of microparasitic and macroparasitic infections, emphasizing the role of transmission parameters, immune responses, and population regulation. The article presents mathematical models that explain how infectious diseases can influence the growth rates of host populations, using laboratory experiments on mice as a case study. It highlights the importance of infectious diseases in regulating wild mammal and bird populations, and discusses the evolutionary relationships among transmission parameters. The article also addresses the implications of indirect transmission and the broader ecological and evolutionary significance of infectious diseases. The first part of the article introduces the mathematical models used to analyze infectious diseases, while the second part, to be published separately, extends these models to include indirectly transmitted infections and their general implications. The article concludes by emphasizing the role of immune responses in determining the population consequences of infectious diseases and the importance of understanding these dynamics for predicting and managing disease outbreaks.