This textbook provides a broad introduction to the biological processes underlying infectious diseases in a range of hosts and pathogens. The text covers topics at all levels of biological organization, from the molecular and cellular level, organismal level, and population and ecosystem level, and goes well beyond infectious diseases of humans. The details of how microbes interact with their hosts are unique for each interaction, but emphasis is on the common principles of host-pathogen interactions that result in disease. Biology of Infectious Disease: From Molecules to Ecosystems is aimed at undergraduate and early graduate-level students in biology or public health, including pre-medical and pre-public-health students, who are interested in a broad introduction to infectious disease but do not have any previous background in microbiology or immunology.