The microorganisms, or microbes, that can cause disease come in different forms. Viruses and bacteria are probably the most familiar, but fungi, protozoa, and helminths are also big players in the story of infectious disease. Our “war” on infectious microbes has restricted the spread of several pathogens and drastically reduced the burden of human disease. But we are a long way from conquering infectious diseases. They account for about one-quarter of deaths worldwide and in 2015 they caused more than half of the estimated 5.9 million deaths in children under the age of 5. Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function. Our bodies often respond with fever, with the secretion of a chemical called interferon, or by marshaling the immune system’s antibodies and other cells to target the invader.