Soap is one of the simplest and most tools for preventing disease. Its molecules have two : one end is attracted to , and the other is attracted to oils and fats. Many viruses are by a thin, fatty layer. Soap this layer apart, and the virus is then away. For this reason, health experts washing hands for at twenty .

In the 1840s, a Hungarian doctor named Ignaz Semmelweis worked at the General Hospital in Vienna. The hospital had two maternity clinics. In the first, run by doctors and medical students, many more mothers died of fever after childbirth than in the second, run by midwives. Women were so afraid of the first clinic that some begged to be sent to the second.

Semmelweis searched for an explanation. He noticed that doctors often came directly from examining dead bodies to examine women giving birth. When a colleague died after being cut during an autopsy and showed similar symptoms, Semmelweis concluded that particles from dead bodies were being carried on doctors’ hands.

In 1847, he required doctors to wash their hands in a chlorine solution. Deaths in the first clinic fell dramatically. Yet many doctors rejected his ideas. Germ theory did not yet exist, and some were offended by the suggestion that they themselves were causing deaths.

Semmelweis died in 1865, with his work still largely unaccepted. Only in later decades, after the research of Pasteur and others, did the medical world recognize that he had been right.