An astronomical number of microbes live in and on our bodies. They are normally harmless and help our body perform vital tasks, but some can cause disease, spread from person to person, and even result in full-blown epidemics. Infectious diseases are caused by bacteria, viruses, fungi and parasites that infiltrate the body’s natural barriers and multiply to produce illnesses, which can range from mild to deadly. Although progress has been made to treat many infectious diseases, humankind is constantly vulnerable to a wide array of both resurgent and new microorganisms.