As there is no obvious reason, fancy theories rationalizing this custom have sprung. The most common of these rationales is the peculiar belief that washing hands with soap has saved countless billions of human lives, as it kills harmful bacteria.
... Keeping hands clean through improved hand hygiene is one of the most important steps we can take to avoid getting sick and spreading germs to others. Many diseases and conditions are spread by not washing hands with soap and clean, running water. http://www.cdc.gov/handwashing/
The intellectual foundation of such tenets, as far as I can see, is that soap detergents weaken lipid membranes, which is, actually, correct. However, this is neither here nor there. Bacterial membranes are made of cross-linked murein polymer (peptide + sugar). Gram positive (GP) bacteria have thick murein coats arranged like chain mail, while Gram negative (GN) bacteria have thin murein coats complemented with an outer (soft) membrane, made of lipids and proteins. It may appear that GP bacteria should be more resistant to soap detergents, which remove these lipids and denature surface proteins, but this is not quite so. GP bacteria rely on passive defenses of their thick murein membranes. Given enough time, soap detergents peel them off one by one, like an onion, and the bacterial cell dies. GN bacteria pursue a different strategy: they have efficient efflux protein pumps that pump out harmful molecules back to the environment. This includes pumping out of antibiotics, toxins, detergents, and staining dyes. “Gram positive” refers to dye staining: GN bacteria can't be stained with certain dyes because they pump them out. GN bacteria are not too afraid of soap detergents, as they can rapidly pump them out. There is damage to their outer coats, but they can counter it by making sugary biofilms.
Most of our gut bacteria are GN bacteria that are not affected by soap at all. Some of them can even live on soap detergents, metabolizing them! Soap solution (after a long contact) kills about 50-80% of GP bacteria, but that only gives GN bacteria a temporary edge over their GP sisters. As GP bacteria multiply in geometric progression, such gentle decreases in numbers are quickly repaired. I've read that soap dispensers are the reliable pathways for spreading gastric infections by GN bacteria. The soap does nothing to these GN bacteria while hundreds of people touch these dispensers. Most of bacteria on our skin that are killed by these soap detergents are harmless. Those surviving tend to be pathogenic, as they are under the evolutionary pressure to resist detergents and carry on infecting people. Even if these GP bacteria cannot survive soap, their spores can. The counts of some bacteria, actually, increase after washing hands, because benign skin bacteria killed by soap can no longer keep them in check. It is an ecosystem.
At this point, someone would interject: what about the flu? Flu virus can indeed be destroyed by soap, but it is one of the least robust viruses. Nucleic acids of viruses are packed in protein capsids; some viruses also have the external envelope of sugar-coated proteins and lipids, which they steal from our own cells. Flu virus are dressed with human lipids, and the soap weakens this envelope; the capsid proteins are quickly destroyed w/o this lipid protection. This is not normal for viruses. The reason it works (why such unstable capsid proteins are tolerated) is that the main mechanism for spreading flu is not by surface contagion, but by aerosols. The soap gets the virus for the same reason it degreases the skin, as both actions involve the same lipids, the human ones.
Soap detergents are just one of many environmental factors that determine microbial winners and losers. Had the soap detergents become an impassable barrier to a pathogen, it would quickly develop resistivity to these detergents, as the biochemical toolbox for such resistivity already exists. The lethality of soap is low and the action is very selective; it does not interfere with the main transmission pathways of the majority of the pathogens, so they do not need to develop such resistance. This is the only reason why soap “works” at all.
I am not talking anyone out of using it, but it is largely a ritual, unless one uses bactericides and/or alcohol based sanitizers. The end result of all this obsessive handwashing activity is that faecal bacteria are present on 25% of hands in Britain, with 11% of these hands containing more such bacteria than the surface of a toilet bowl. http://www.bbc.co.uk/news/magazine-1983
Since numerous pseudoscientific “reasons” have been invented to wash hands, one may ask: what is the actual basis for our cleansing ritual?
I know of no such basis than the commandment to clean one’s hands. It gives me feats of laughter when someone who just a minute ago grieved over my “irrational beliefs” excuses himself to wash hands, rubbing a soap bar against his skin. I find it hard to continue a high-spirited discussion of human rationality after such grotesque displays.
Perhaps this is the chief reason that we are commanded to wash our hands: to rub this truth in. Every time you hear a bogus rationale for an act that has no other reason than one’s faith, you are reminded of what self-deceiving beings we are. All of such rubbish needs to be cleansed away before one can hope to enter the house of Reason.
Blessed be the one who commanded us to wash hands.
בָּרוּךְ אַתָּה הָ׳ אֱלֹהֵינוּ מֶלֶךְ הָעוֹלָם אֲשֶׁר קִדְּשָׁנוּ בְּמִצְוֹתָיו וְצִוָּנוּ עַל נְטִילַת יָדָיִם