This is the eBook version of the printed book. This Element is an excerpt from Germs, Genes, & Civilization: How Epidemics Shaped Who We Are Today (9780137019960) by David P. Clark. Available in print and digital formats.¿ Meet the “opportunists”: the microbes that are poised on the edge of invading you right now. ¿ Consider disease from the microbe’s viewpoint. Infectious agents vary greatly in their ability to cause harm. Before discussing “professional” diseases, don’t forget the “opportunists.” When a person is weakened by injury, exposure, or starvation, or if the immune system is malfunctioning, otherwise harmless microbes may cause disease. Such opportunistic diseases have received much attention in connection with AIDS.