This question better suits /r/askscience, since the answer you're looking for is based on the molecular structure of soap. However, let me give you an answer regarding the way soap works and why "we got really lucky". (I hope this doesn't break the rules of the sub, since it's not really focused on history. If it does break the rules, please delete.)

Soap can be produced through different processes and with different ingredients, but virtually all soap has one thing in common: the individual soap molecules have a hydrophilic (water-loving) & lipophobic (oil-fearing) head and a lipophilic (oil-loving) & hydrophobic (water-fearing) tail. This molecule structure enables soap and water to remove oily substances from surfaces, which water alone can not clean.

The lipophilic tails of the soap molecules bind to individual oil molecules covering the surface and, in doing so, break bonds between the oil molecules. The hydrophilic tails, on the other hand, bind to the molecules of the water used in the cleaning process. Rinsing the soapy surface with water removes those bonded water-soap-oil molecules. Rubbing or brushing the soapy surface before rinsing, increases the ability of the soap molecules to break the bonds between the oil molecules.

This cleaning process is and has been useful for all kinds of circumstances, including hygiene, but mainly the cleaning of tools and other objects of use.

Now, here is where "we got really lucky". Many bacteria and viruses, including the Corona virus family, have a lipid layer in their outer membrane. These lipids are exactly what the lipophilic tails of the soap molecules like to bond to and, in doing so, they do exactly what they do on an oily surface: they break the bonds between the lipid molecules and thereby destroy the membranes.

However, not all bacteria and viruses have an outer lipid membrane. E.g. gram positive bacteria have a cell wall around the membrane. Those bacteria and viruses may not be destroyed by the soap but simply removed from surfaces (our bodies) by breaking the bonds in the oily film (on our skin or on tools) to which they stick.

I feel like this question assumes that the important part of soap is it's ability to kill microbes, which is a widespread incorrect assumption. Soaps primary role in cleaning you is as a surfactant. This is the same process that cleans your pan, and it's what leaves our hands largely germ free.

When you wash, your goal isn't to kill all the harmful bacteria, it's to remove them from you. Soap acts as an intermediary between contaminants and water. Most famously between oil and water. It creates a bond with something water won't bond to and also bonds to water. Then you can rinse those contaminants down the drain with the soap.

So in that sense, no it wasn't an accident. Soap works on the same principal that it was invented for when it's disinfecting your hands. Antibacterial labels on soap are largely marketing. Here in the hospital we use antibacterial soap in some important places, but they don't do the heavy lifting. If you want cleaner hands it's not really effective to get stronger soap. What's effective is scrubbing more and scrubbing harder, because disinfecting your body is mostly a mechanical process.

If you have an open wound that can't be scrubbed, that's when antimicrobials become more important. But even then they don't work nearly as well as good old elbow grease.

Source: I'm an engineer in a hospital and I've taken more education hours on soap, infection mitigation, and hand washing than really makes sense.

Soap was invented thousands of years before we understood germs, why? A fun question, because it points to larger cross-cultural notions: that of cleanliness, bathing, washing, and purity, all have (probably) existed deep in human history. These ideas are core elements of so many ancient belief systems, all of which are emerged prior to the knowledge of microorganisms and lipid layers.

People want themselves and their clothes clean, and peoples in North America used various plants for the purpose. I'll repost a section about cleanliness in my post on indigenous North American medicine...

Habitual washing and cleanliness was certainly a health improvement compared to contact-period Europeans. The daily or at least common washing of oneself was done both as act of purification, but was also understood to be for cleanliness; and this was done across the Americas. People on the northern plains would wash themselves daily even in the winter. Hot springs and steam (in sweat-lodges) were used to treat medical problems such as gout, arthritis, rheumatism, or a difficult labor. And some peoples used medicinal herbs in those sweat-lodges to treat problems, particularly joint pains using the herb boneset.

Yet cleanliness did not stop there. Various peoples made soaps and shampoos out of particular plants, such as utilizing the unsurprisingly-named soap plant. The Karuk used processed bulbs of the plant, though the Mahuna used whole bulbs as bars of soap. The Kumeyaay and Kawaiisu only used the roots of these plants. These peoples additionally used that plant for shampoos. Great Basin peoples used varieties of yucca as a body wash and shampoo, whereas the Cherokee used bear grass roots as a detergent. Wild gourd or buffalo gourd was used by the Kumeyaay and Ivilyuqaletem (Cahuila) as a laundry detergent and as bleach. The Tohono O’odham and Puebloan peoples washed their clothes with the wild gourd. Sometimes the gourd was cut in half and rubbed on the clothing as a stain remover.

Consequently for many societies, keeping oneself clean became associated with spiritual purity. As G. Goodwin states about the Inde (Western Apache) in the 1930's, *"Bodily cleanliness and new clothes were almost prerequisites for participation in certain ceremonies to gain approval of the supernaturals. 'Unless you take a bath and dress nicely, you will smell bad and they will not like you,' it was said."*^1

J. Mooney notes for the Tsalagi (Cherokee) in 1891 that "going to water" (bathing) was a frequent ritual which was both sacred and medicinal. It had to be done to cleanse oneself for important ceremonies: Before various dances, before eating at the Green Corn dance, and both before and after playing stickball. It was also done to mark the cycle of time by moonths (moon-months), as it was done at each new moon. Aside from its grand ceremonial use, it was "regular treatment" for various diseases, and done for psychological or magical medicine - to help cure "bad dreams" or to fight an enemy's curse/spell against you. It was even done along with "prayers for long life."

In these societies, bathing was both a common medical procedure and a part of a yearly cosmic ceremonial cycle. The importance and dual use of bathing (using a sweat lodge) was noted by early European visitors like Rodger Williams who wrote in 1643 about local Algonquian speakers in Rhode Island, *"They use sweating for two ends: first to cleanse the skin, secondly to purge their bodies, which doubtless is a great means of preserving them."*^2

Their near-contemporaries to the south the Aztecs would agree. They used stone circular sweat lodges called temazcal (bathing-house) and they were both medicinal and spiritual. As Sahagun notes (early 1500's), sweating was done medically: to treat fractured bones, syphilis, leprosy, chest/back pains, skin spots/growths, and was done before and after giving birth, and for other medical issues. Yet these treatments were done under the protection of the goddess of the sweat bath Temzcalteci, as Sahagun notes, "this goddess was the goddess of medicine and of medicinal herbs. She was adored by doctors, surgeons, and bleeders, and also by midwives." And so entering the temazcal was as if the bather had entered the goddess' womb, so it was a sacred space as well.^2

As Mooney notes, the details of 1890's Tsalagi bathing ceremonies are "very elaborate and vary according to the purpose..." but all situations require a medicine worker and both the worker and bather fast for a day before. It is performed at daybreak, a reference to the power of the sun, and the bather "dips completely under the water four or seven times..." One submergence for each of the four directions, and sometimes one for all that is above, all that is below, and one for the center: oneself. In North America, sweat lodges often faced east, again a reference to the power of the sun.^2

Steam baths are an ancient practice in the Americas, first seen at Olmec sites in southern Mexico at Teopantecuanitlan. The generally contemporary site of La Venta has thrones shaped as steam baths and show midwives, all of this referencing the king being spiritually born (and gaining authority) from the womb-throne of a steam bath.^3 This was around 2000 years before historical records of the Nahua goddess Temzcalteci. Such powerful and sacred places became sites of pilgrimage, at the Guatemalan Mayan site of Xultun, a sweat bath was made ca. 300-600 CE which was shaped as a "female amphibian deity." Eventually it was buried, only to be unburied (in some capacity) hundreds of years later (ca. 900 CE) so that it could be used as a pilgrimage site as offerings were left at it.^4

In North America, Timothy Pauketat mentions that sweat lodges as they are known today, are first seen at Cahokia (near St. Louis Illinois). This Mississippian period site which flourished ca. 1050-1300 CE was also the center of a novel religious society and presumably sweat lodges were a major part of this tradition. As Pauketat suggests, circular steam baths were possibly influenced from the observation of moon rings (the 22 degree halo), which appears around the moon and bring (about 2-3 days later) a warm front and rain.^5,6 Outside of North America, sweat baths are a "typical part" of midwifery around the world;^7 being used for traditional healing in places as far-afield as modern Sierra Leone^8 and early modern Ireland (the multi-purpose cooking/dyeing/bathing structures called fulacht fiadh).^9

Vapour baths were in use among the Celtic tribes, and the sweat-house was in general use in Ireland down to the 18th, and even survived into the 19th century. It was of beehive shape and was covered with clay. It was especially resorted to as a cure for rheumatism.

Back in North America, the blending of cleanliness and purity influenced social values; resulting in disdain for those who didn't follow these social trends. For 1930's Inde, "Those who permitted their hair to go uncombed and wore dirty clothes were considered lazy and consequently poor..." But even then, there were some who didn't follow the normal social values, as "the pride taken in personal attire varied considerably; some dressed showily, and others did not." Aside from being lazy and poor, Goodwin notes that some chiefs "...almost always dressed in old clothes," so if you had a particular social influence you could flout the rules!