Why does washing your hands stop disease?
― The doctor who first noticed it, nobody believed
"Wash your hands when you get home." This habit, taken for granted today, wasn't even common sense among doctors just 180 years ago. The first person to prove with numbers that handwashing saves lives was a young doctor working in a hospital in Vienna. What he discovered was that the source of disease could be "carried on the hands."
You come home from outside and look at your hands. No dirt, no grease. They look clean, so you reach for a snack without washing them.
But those hands had just been touching train straps, doorknobs, and money. Nothing shows up to the eye.
"Dirty, even though you can't see it." This idea itself was hard for people in the past to accept.
There are only two reasons handwashing prevents disease
Many diseases happen when tiny creatures invisible to the eye (bacteria and viruses) get into the body. They can't travel far on their own. Hands carry them from whatever they touched to wherever they touch next.
These tiny creatures cling to the oils and grime on your skin. Soap breaks that oily film apart and floats it in water. Running water then carries it off your hands.
Today, these two facts are taught in elementary school. But the doctor who first noticed them was barely taken seriously by his colleagues. Let's start with that story.
Same hospital, but a threefold difference in death rate
In the 1840s, the Vienna General Hospital in Austria had two maternity wards. A high fever that struck after childbirth and often killed the mother, called "childbed fever" (puerperal fever), was a major problem at the time.
Ignaz Semmelweis, a young doctor working at the hospital, noticed something strange. In the First Clinic, staffed by doctors and medical students, roughly 1 in 10 mothers died. In the Second Clinic, staffed by midwives, it was closer to 1 in 30. Compare the two left-hand bars in the right-hand graph of Figure 1.
The townspeople knew about this gap too. Some pregnant women are said to have wept and begged not to be admitted to the First Clinic. Many doctors blamed crowding or bad air, but the two wards were in the same building, under nearly identical conditions.
The turning point was a colleague's death
In 1847, Kolletschka, a doctor senior to Semmelweis, died. He had been cut on the finger by a student's scalpel during a dissection. His symptoms were strikingly similar to those of the mothers dying of childbed fever.
This is when Semmelweis landed on the difference between the two wards. Doctors and students in the First Clinic dissected corpses in the morning, then went straight on to examine mothers in labour. Midwives never performed dissections. He reasoned that some "something" from the corpses was staying on the doctors' hands and entering the mothers' bodies.
In May 1847, he ordered everyone entering the First Clinic to wash their hands in chlorinated water (a solution of chlorinated lime). It had a strong smell and was rough on the skin. Even so, the effect was unmistakable. The following year, 1848, the First Clinic's death rate fell to about 1.3%. That's the rightmost bar in Figure 1.
Semmelweis didn't know the source of disease was a tiny living thing. He called it "cadaveric particles." It wasn't until the 1860s–1880s that Pasteur and Koch showed germs cause disease. Without understanding the mechanism, he had arrived at the right answer through numbers alone.
What exactly do soap and running water remove from your hands?
Today we know what Semmelweis couldn't see. Skin always carries a thin film of oil. Bacteria and viruses picked up from whatever you touched cling to that oil and grime. Water alone struggles to remove it, because it repels oil.
A soap molecule has one end that's drawn to water and one end that's drawn to oil. The oil-loving end burrows into the oily film and surrounds it, forming tiny droplets. Since the outside of each droplet is water-loving, running water carries it away from the hand.
What's more, many cold and flu viruses are wrapped in an outer layer of fat. Soap is thought to break down this envelope itself. In other words, handwashing's main job isn't "killing" germs so much as "lifting them off and washing them away." Both the rubbing and the rinsing matter.
The claim that "doctors' hands are carrying disease" meant admitting that doctors themselves were the cause of harm. There was also no mechanism yet to explain why. Semmelweis left Vienna and died in 1865 at age 47. His ideas only gained wide acceptance later, once research into germs had advanced.
So what should you actually do?
- Work up a good lather and scrubWater alone repels oils, so whatever's stuck tends to stay behind.
- Fingertips, nails, thumbs, all the way to the wristsFingertips and the base of the thumbs are said to be the most commonly missed spots.
- Scrub for at least 20 seconds, then rinse thoroughly under running waterThe US Centers for Disease Control and Prevention recommends 20 seconds or more — about the length of humming half a song.
Summary
Handwashing prevents disease because invisible sources of disease are carried on the hands, and soap and running water lift them off and wash them away. The first person to show this was a single doctor who kept comparing the numbers from two wards, without ever understanding the mechanism.
Invisible doesn't mean clean.
Handwashing is a life-saving habit that numbers taught us 180 years ago.
You can read more about how soap wraps up oil in "Why don't water and oil mix?", and about how fast bacteria multiply in our article on curry left out overnight.
- Spread a little hand cream or cooking oil on your palm, then dust on about half a teaspoon of cinnamon or similar powder, spreading it evenly.
- Wash as usual with just water for 10 seconds. Then try lathering soap and scrubbing for 20 seconds before rinsing.
- After each wash, compare your fingertips, around your nails, and the base of your thumbs under bright light to see where powder remains.
The powder stands in for germs as a visible marker. You'll see that with water alone, powder tends to stay behind along with the oil. If the powder bothers your skin, skip this experiment.
Want to go deeper? ― terms, formulas, and textbook linksWe mark clearly which level each part belongs to, from middle-school science to university specialist courses
- MScovered in middle-school science
- HScovered in high-school biology or chemistry
- HS+advanced high-school content, or textbook sidebar material
- Univcontent not taught in high school — university-level specialist subjects (epidemiology, microbiology)
- Researchnot yet settled as "established fact" even at university — what researchers are actively investigating
MSTerms: this phenomenon has a name
- Childbed fever (puerperal fever): an infection after childbirth, caused by bacteria entering through wounds in the birth canal. In the era before antibiotics, it could be fatal.
- Surfactant: a substance whose molecule has one water-loving part and one oil-loving part. The main ingredient in soap and detergent.
- Contact transmission: disease passing from person to person via hands or objects. This is mainly the route handwashing blocks.
MSHSCheck with a formula: how many mothers did handwashing save in a year?
From the numbers Semmelweis left behind, we can estimate the death rate and the number of lives saved by handwashing.
| In symbols | p = D ÷ N , S = N × ( p_before − p_after ) |
| In words | Death rate = number of deaths ÷ number of births. Lives saved = number of births × (death rate before handwashing − death rate after handwashing) |
| Where it comes from | The basic epidemiological definition of a "rate." If the same number of people die at the before-rate and the after-rate, the difference is the number of deaths prevented by handwashing |
| Symbol | Meaning and unit |
| N | number of births (cases) |
| D | number of mothers who died (people) |
| p | death rate (ratio, no unit) |
| S | estimated lives saved by handwashing (people) |
| Number of births in 1st Clinic (6 years, 1841-1846) | 20042 cases |
| Mothers who died in the same period | 1989 people |
| 2nd Clinic death rate (same period) | about 3.38% |
| 1st Clinic death rate after handwashing (1848) | about 1.27% |
| 1st Clinic death rate before handwashing | 1989 ÷ 20042 ≒ 0.0992 |
| How many times the 2nd Clinic's rate | 9.92 ÷ 3.38 ≒ 2.9 |
| Drop in death rate from handwashing (%) | 9.92 − 1.27 = 8.65 |
| Births per year | 20042 ÷ 6 ≒ 3340 |
| Estimated lives saved per year | 3340 × 0.0865 ≒ 289 |
③ To put that in perspective: simply starting handwashing meant that in just one ward, about 290 mothers a year stopped dying. That's roughly as many people as several elementary-school classes, protected every single year.
HSHS+Soap molecules, and what chlorine does
HSSoap is a sodium salt of a fatty acid, with a long hydrocarbon chain (the oil-loving part) and a carboxylate ion head (the water-loving part). At higher concentrations, the molecules cluster into "micelles" that trap oil inside.
HS+The chlorinated lime solution (containing calcium hypochlorite) that Semmelweis used breaks down microbial proteins and other matter through oxidation. Where soap "lifts off and washes away," chlorine works by "destroying." Because it's harsh on skin, today's hospitals mainly use alcohol-based hand sanitizers instead.
UnivChecking by comparison ― the logic of epidemiology
Semmelweis's method — comparing rates between two groups and eliminating possible causes for the difference one by one — takes the same basic shape as later epidemiology. The ratio of rates is called "relative risk," and the difference in rates is called "attributable risk"; the 2.9 and the 8.65-point gap in the calculation above are examples of each. The idea that disease is caused by microorganisms is known as "germ theory," established through the work of Pasteur and Koch. Efforts to maintain the benefits of handwashing across entire hospitals are now compiled by the World Health Organization under the heading of "hand hygiene."
📖 Going further: Semmelweis's life and records (Wikipedia, Japanese) / Childbirth and childbed fever (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- How many seconds of washing are enough? Guidelines of 20 or 40 seconds exist, but how well dirt comes off varies greatly with the type of grime and the washing method, and the optimal length is still being studied.
- Pathogens alcohol struggles against. Viruses without a fatty envelope, such as norovirus, are said to be less affected by alcohol. Research continues into when to use soap versus alcohol.
- The relationship with bacteria that already live on your hands. Skin is also home to bacteria that help protect the body. How over-washing or damaged skin changes that protective role is still not well understood in many respects.
In other words, even this article describes things "as currently understood." Just as Semmelweis reached the right answer without understanding the mechanism, science sometimes finds "what to do" before it finds "why."
Links to textbooks (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science, biology - body structure, microorganisms | Reason 1 - disease carried by hand |
| HS | Chemistry - fats, oils and soap | How soap molecules wrap up oil |
| HS+ | Chemistry - oxidation and reduction | How chlorine solution destroys microbes |
| Univ | Epidemiology, microbiology | Relative risk, germ theory |
| Research | Infection control, skin microbiology | Wash time, choosing methods, skin flora |
| ― | Everyday connections | Handwashing after coming home, before meals, after the toilet |
- Ignaz Semmelweis, The Etiology, Concept, and Prophylaxis of Childbed Fever (1861, English translation K. Codell Carter, 1983, University of Wisconsin Press)
- Wikipedia "Ignaz Semmelweis" (Japanese: センメルヴェイス・イグナーツ)
- World Health Organization (WHO) - World Hand Hygiene Day
- WHO Guidelines on Hand Hygiene in Health Care (2009, World Health Organization)
- Sherwin B. Nuland, The Doctors' Plague: Germs, Childbed Fever, and the Strange Story of Ignác Semmelweis (2003, W. W. Norton)
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanisms. Historical figures are based on records from the time and may vary slightly between sources. If you have health concerns, please consult a medical institution, and during disease outbreaks, follow the guidance of public health authorities and local government.