Since humankind's earliest days, the rise of civilization has led to a huge increase in the number and types of diseases that afflict the human population. Early theories for the cause of ailments were based on superstition, religion, or imbalances in the body. Without an accurate understanding of what causes sickness, doctors treated their patients with herbal remedies or medical procedures that would often worsen a patient's condition. It was not until later, when scientists such as Antony van Leeuwenhoek and Louis Pasteur studied bacteria and viruses, that advancements could be made in preventing and curing widespread diseases. Germ Theory examines the major epidemics in world history, the work of major scientists who helped develop the field, and the potential global impact of drug-resistant germs.