Over the past month, social media has been full of explainers and how-to stories related to the coronavirus. As always on the internet, the trick is figuring out which sources are reliable and which are made up of guesses and wishful thinking. Do you really need to wash your fruit with soap and water? Is it enough to stand six feet away from another person, or can sneezes transmit the virus up to 27 feet?But the one I keep thinking about is the widely reported study in the New England Journal of Medicine about how long the coronavirus can survive on different surfaces. Apparently, the virus lingers on copper for four hours, on cardboard for 24 hours, and on plastic for up to 72 hours. What it all means in practical terms is unclear—the study didn’t say whether the virus was still infectious after all that time, just that it was still detectable. But it’s enough information to set off a chain of obsessive calculations. When did I bring home that plastic water bottle? What about the paper bag it came in? If I touched the bottle and then the bag, could I have transferred the virus from one to the other?It all felt strangely familiar, and after a few days I realized why: This is exactly the way the Talmud talks about tumah, ritual impurity. For the rabbis, tumah acts just like a virus: It’s an invisible, intangible, but highly communicable vector of contamination, which can jump from person to person and surface to surface. That NEJM study could almost be spliced into the portion of Tractate Pesachim known as the sugya of Rabbi Hanina Segan HaKohanim (“Hanina the Deputy High Priest”)—a stretch of seven pages in which the rabbis discuss in minute detail which substances transmit impurity and in what degree. Here, for instance, is a gloss on Pesachim 17a, describing a hypothetical chain of transmission of tumah:“The corner of the garment comes into contact with a dead creeping animal, conferring upon the garment first-degree ritual impurity status; bread comes into contact with the garment, conferring upon the bread second-degree ritual impurity status; stew comes in contact with the bread, conferring upon the stew third-degree ritual impurity status; and wine comes in contact with the stew, conferring upon the wine fourth-degree ritual impurity status; and oil comes into contact with the wine. In that case, does the wine confer upon the oil fifth-degree ritual impurity?”You might say that the pandemic is turning back the clock on the way we think about contamination—a long way back, since tumah hasn’t played a major role in Judaism for almost 2,000 years. That’s because the concept of ritual purity was completely bound up with the Temple: Impure priests couldn’t enter the Temple to perform their duties, and only Temple offerings could purge certain kinds of impurity. Since the Temple was destroyed in the year 70 CE, later halachic authorities ruled that over the generations all Jews became tamei, impure, and will remain so until the Temple is miraculously rebuilt. As with the coronavirus, there is no cure or vaccine for tumah.But even if most Jews no longer care about tumah or even know the word, the idea exerts a subterranean influence. The translation “ritual impurity,” which is used by most sources today, makes tumah sound irrelevant to the qualities we now think of as essentially religious, such as holiness or goodness. The King James Version of the Bible, however, gives a better sense of its ambiguous power by translating tamei as “unclean,” as in Leviticus 7:21:“And when any one shall touch any unclean thing, whether it be the uncleanness of man, or an unclean beast, or any unclean detestable thing, and eat of the flesh of the sacrifice of peace-offerings, which pertain unto the Lord, that soul shall be cut off from his people.”The word “unclean” has moral overtones, and as this passage suggests, being tamei was no abstraction. It could result in ostracism, excommunication, and even eternal punishment: The Hebrew verb translated here as “cut off” is karet, which the Talmud uses to describe the severing of the soul from God after death, the worst punishment known to Jewish law.What’s most puzzling about tumah is that such dire consequences could result from a condition that has nothing to do with conscious choice. Like a virus, impurity isn’t something you do but something you catch. The chief sources of tumah in Jewish law are lepers, corpses, semen, and menstrual blood, and the carcasses of certain animals; a person who comes into contact with these is called an av hatumah, literally a “father of impurity,” who can transmit the condition to others.And the methods of transmission aren’t always obvious. For instance, you don’t have to actually touch a corpse to become tamei; just being under the same roof as a dead body is enough. Metal instruments like spoons can become tamei, but earthenware vessels can’t. This may sound arbitrary and illogical, but then, until the discovery of germ theory by Louis Pasteur and Robert Koch in the late 19th century, “scientific” theories about the spread of disease weren’t much better. Our word “flu” comes from the Italian influenza, referring to the malign influence of the stars, which was supposed to cause the disease. As late as the 1850s, most doctors believed that cholera and other infectious diseases were caused by “miasmas,” vapors generated by decomposing matter.Now that we have accurate scientific knowledge about viruses and how they spread, we are much better off than our ancestors in terms of our ability to combat infectious diseases. But psychologically, we are in pretty much the same boat as Hanina Segan HaKohanim, 2,000 years ago—trying to keep track of what’s clean and what’s unclean, what item touched which surface and when. A few weeks ago, thinking this way would have been considered a symptom of obsessive-compulsive disorder. Thanks to COVID-19, it now feels sadly like common sense.