One of the best ways to prevent the spread of the flu and other viruses is to wash your hands. Today, this may seem like common sense to many people (even if they don’t all do it properly). Yet it wasn’t until the mid-19th century that some doctors in the United States and Europe began to wash their hands before examining patients—and even then, only in certain cases.

An early proponent of hand washing was Ignaz Semmelweis, a Hungarian doctor who worked at the Vienna General Hospital between 1844 and 1848. The hospital was one of the largest in the world for teaching, and its maternity wing was so big that it was divided into two wards: one for doctors and their students and one for midwives and their students.

Yet there was a stark disparity between these wards.

Between 1840 and 1846, the maternal mortality rate for the midwives’ ward was 36.2 per 1000 births, while the mortality rate for the doctors’ ward was 98.4 per 1000 births, according to a 2013 article in the Journal of the Royal Society of Medicine. Specifically, the doctors’ ward had a higher rate of “childbed fever,” now known as streptococcal infection. Semmelweis started to look for any differences between the wards.

One difference was that in the doctors’ division, a priest regularly passed through and rang a bell as a last sacrament to the dying women, explains Dana Tulodziecki, a philosophy professor at Purdue University who has written about Semmelweis in the journal Philosophy of Science. Semmelweis wondered if women were dying because of “the psychological terror of hearing the bell—so even if you’re not actually dying, you just hear the bell, you know it could be your time.” Semmelweis rerouted the priest, but it made no difference.

Then in 1847, the death of Semmelweis’ colleague Jakob Kolletschka led him to a breakthrough. Kolletschka had cut his finger on a scalpel during an autopsy, and developed an infection that killed him. Semmelweis wondered whether a similar type of infection could be happening in the doctors’ maternity ward.

Semmelweis realized that, unlike the hospital’s midwives, doctors sometimes examined women in the maternity ward after performing autopsies. In the absence of germ theory, Semmelweis theorized Kolletschka had died because “cadaveric matter” entered his body through his wound, and that women in the doctors’ ward might also be dying because cadaveric matter from doctors’ hands was entering their body through their genitalia.

Although this was incorrect, Semmelweis’ response to his theory was pretty good. He started mandating that doctors wash their hands with chlorinated lime after autopsies. And it was a big improvement—between 1848 and 1859, the maternal mortality rate in the doctors’ ward dropped to around the same level as the midwives’ ward.

After this, the story becomes a little controversial. Previous scholars have argued Semmelweis tried to convince other hospitals to adopt his policies, and that they refused. Tulodziecki says the real story is more complicated. Yes, “doctors weren’t pleased that Semmelweis essentially implied that they were responsible for killing all these women,” she says. Yet “it’s also true that when he finally did publish the etiology of childbed fever, it wasn’t very well-written; it’s kind of rambling in parts. He was also a really stubborn person, very dogmatic.” As she says, "Overall, he could have made his arguments better."

Semmelweis insisted all childbed fever was caused by cadaveric matter or decomposing animal matter, which didn’t make any sense. Childbed fever was a very old infection that appeared in home births as well as the midwives’ ward at Vienna General Hospital, where cadaveric or decomposing animal matter wasn’t a factor. Making sure doctors washed their hands after autopsies was one way to reduce childbed fever, but Semmelweis alienated his colleagues by insisting it was the only way—which didn't seem likely to them.

In any case, Semmelweis wasn’t the only doctor in the mid-19th century to realize medical professionals’ own hygiene might have some effect on their patients. In 1843, the American doctor Oliver Wendell Holmes published a paper arguing doctors with dirty hands could cause childbed fever in their patients. The British nurse Florence Nightingale, considered the founder of modern nursing, wrote in her 1860 publication Notes on Nursing that “Every nurse ought to be careful to wash her hands very frequently during the day.”

Still, the importance of hand washing for medical professionals didn’t really become understood until scientists hit upon germ theory—the idea that certain diseases and infections are caused by microorganisms we can’t even see. In particular, the British surgeon Joseph Lister drastically improved patient mortality by advocating that surgeons wash their hands and sterilize their instruments in between patients.

Today, medical and health professionals consider hand washing a critical hygienic practice, both for themselves and their patients. The Centers for Disease Control and Prevention, or CDC, even provides guidelines for how to properly wash your hands. To properly kill germs, the CDC advocates scrubbing them with soap for at least 20 seconds before rinsing the soap off with water. Drying them completely is also important, since wet hands spread germs more easily.