Microbiology is the branch of biology dealing with the smallest of living things: bacteria, fungi, algae, protozoa, and viruses. Immunology is the study of the response of higher organisms to foreign substances, including microbes. Because of their rapid growth rate and relative simplicity, microbes are often the experimental subjects of choice for examining basic genetic and biological phenomena. A significant fraction of contemporary biochemical research employs microbiological and immunological methods. Some research areas in which practical and theoretical advances are occurring include: the study and evolution of microbial species pathogenic to animals, plants, and humans; genetic engineering of microbes to analyze basic biological processes and generate valuable products; the nature and occurrence of microbial life in extreme or unusual environments; the role of microbes in stabilization of the biosphere by recycling and detoxifying waste products; and the genetics and regulation of the immune response.