Discoveries of the existence of cells (Hooke) and little animalcules (Van Leeuwenhoek) initiated a series of investigations that eventually led to the birth of Microbiology.
Germ Theory of Disease considers the possibility that microorganisms might have similar relationships with plants and animals - that microorganisms or "germs" caused disease.
Edward Jenner discovered that a milkmaid who previously contracted cowpox (mild form) was not afflicted by smallpox (severe form) and collected scrapings from cowpox blisters to prepare pox-contaminated needles.
Recent outbreaks point to the fact that infectious diseases are not disappearing, with new diseases appearing or resurging such as H1N1 Influenza, Avian H1N1 Influenza (Bird flu), MRSA (Methicillin resistant Staphylócoccus aureus), E. coli, Human Immunodeficiency Virus (HIV), and SARS Corona Viruses (COVID-19).
Antibiotics/chemotherapeutic drugs are not without problems as they can kill the good microbial flora, some can kill pathogenic microbes but can damage infected host cells as well, and some are too toxic for humans and human cells.
An infectious disease is a disease in which pathogens invade a susceptible host, carrying out at least a part of its life cycle inside the host, resulting in disease.
Majority of microbes benefit humans, animals, and plants in many ways by recycling vital elements, recycling water, removing pollutants, and controlling pests.
Biofilms are complex aggregations of microorganisms attached to each other and/or a solid surface, which can be beneficial by protecting the mucous membranes from harmful microbes, but can also be harmful by clogging pipes or infecting prosthetics.
Alexander Flemming observed that culture media containing bacterial colonies but contaminated with the mold Penicillium showed inhibition of growth around the mold.