A recent announcement in the Israeli religious media warned against drinking water during the first hour of summer. The consequences, readers were told, could be fatal.

Though seldom heard and generally unheeded by contemporary observant Jews, this restriction has a respectable place in the history of Jewish practice.

The Talmud does not mention the warning, but it appears in medieval, especially Ashkenazic, halachic sources. While neither Maimonides nor the Sephardic Rabbi Joseph Karo (author of the Shulhan Aruch) codify the rule, Rabbi Moses Isserles (“Rema”) — Karo’s contemporary, who appended the rites of Germany and his native Poland to the Shulhan Aruch — cites the water prohibition as a “widespread custom” that “must not be changed.” (That is a sure sign many Ashkenazim were ignoring it by the sixteenth century.)

The custom’s origins are obscure. It likely derives from a mythology of the seasons that deemed water unsafe at the four annual “tekufot” — the vernal and autumnal equinoxes and the winter and summer solstices. These are fleeting moments between the seasons, when, in the old geocentric model, the sun appears to cross one of four points on the celestial sphere. (We now know that the seasons are caused by a tilted Earth revolving around the sun.)

Why would water turn deadly at regular intervals throughout the year? This tradition thought of the solstices and equinoxes as vulnerable times, at which an otherworldly poison contaminated the water supply. In the words of Jacob ben Samson, a student of Rashi writing in the early twelfth century:

An angel is appointed over each season. But as the new guard replaces the old, the mighty warriors Cancer and Scorpio become filled with jealousy and rage . . . They engage each other in battle . . . hurling venom and wrath. The venom falls into the springs and aquifers whose waters flow to the valleys and hills, into every pool of water on the face of the earth, and into the waters inside wooden and stone vessels. The waters will kill all who drink from them at this hour.

While this explanation may sound fanciful to modern ears, practical rather than metaphysical considerations motivated the water prohibition. In fact, the rule is located within a section of the Shulhan Aruch that deals with everyday hazards. In the same note, Rema lists several safety-related warnings from the Talmud: Do not walk alone at night or near a leaning wall; upon the outbreak of plague within a city, escape immediately; do not place your mouth directly on the spout of a public fountain (for fear of unseen leeches; microbes deposited by previous users were unknown). In Rema’s view, water drawn at the solstice or equinox was another tangible danger prohibited by Jewish law.

Because halachah is meant to encompass the entirety of human life, some of its standard texts reflect obsolete ideas about the natural world.  While anchored in tradition, halachah as practiced is not an antiquarian exercise.  If revived, the water prohibition would amount to the veneration of a relic rather than reverence for Jewish tradition.

In a way, the re-emergence of the water taboo is darkly comical. If the fear of death by drinking water is gaining traction, some continue to deny the real medical risk associated with a dangerous circumcision practice.

An erroneous theory of infection in ancient Greek medicine maintained that stagnant blood decays and may turn to pus. For this reason, in all likelihood, the Mishnah required suctioning after circumcision (“metzitza”). The Talmud does not specify any suctioning method. Before the discovery in the nineteenth century that bodily fluids may carry and transmit deadly pathogens, mohalim would often perform “metzitza be-peh” (direct oral suctioning, or MBP).

In the nineteenth century, when outbreaks of syphilis and other diseases were linked to MBP, leading Orthodox rabbis such as Moses Sofer and Chaim Berlin permitted more hygienic methods. Others, like the renowned Chaim Soloveitchik of Brisk, actively discontinued MBP. They argued that suction was intended in the first place as a safety measure, and was not an integral part of circumcision. (Shlomo Sprecher provided a detailed history in his 2006 Hakirah article on the subject.)

In recent years, among the minority that still practices it, MBP has been linked to the transmission of the herpes simplex virus 1 (HSV-1), resulting in cases of brain damage and death. But instead of urging their followers to abandon the practice, some rabbis have made MBP a rallying cry for tradition while they invoke the constitutional right to religious freedom. The issue remains an unlikely source of contention in New York City politics. (In 2012, the New York City Department of Health required parental consent before a mohel performs MBP; Mayor Bill de Blasio rescinded the rule earlier this year.)

Halachah places health and safety above nearly all other values. It prohibits life-threatening behaviors and rejects the temptation to rely on miracles. The Talmud states that “a danger is more serious than a prohibition”; that is, the possibility of danger, even when a negative outcome is uncertain, carries more weight than the possibility of violating a prohibition.

And herein lies a tragic irony: MBP violates halachah’s abhorrence of unnecessary health risk and upends the priority of safety over ritual. Once thought to protect a newborn from disease, MBP increases that very risk.

A confident halachah has no use for long-forgotten taboos. And in the 21st century, a practice that places infants at greater risk for illness has no place among the rituals of responsible Jews. While abiding by the former is merely foolish, tolerance of the latter will continue to cause needless harm to our children and our community.