You can’t smell it. At first, you can’t taste it. But once you do, you’ll wish you’d never opened the bottle.

It’s a stealthy little pest known as mouse, or mousiness, an off-flavor that can infect wines. Depending on whom you ask, it tastes like a dead mouse, or a caged mouse, or “dog halitosis.” Confusingly, it can also taste like crackers or corn chips.

It’s experienced retronasally, meaning your nose perceives it when it’s in the back of your mouth, as an aftertaste, so a wine that might smell perfectly pleasant and initially taste fine could spoil by the time it arrives at your tonsils.

To those of us who can taste it — as much as 30% of the population may not be able to taste it, according to the Oxford Companion to Wine, and I envy you — the presence of mouse completely ruins a wine. And the bad news: Although its occurrence has been documented since the 19th century, lately it seems to be on the rise.

Mouse is mysterious. Its ways are not entirely understood. But it’s clear that high pH, exposure to oxygen and low levels of sulfur dioxide can all create conditions for mouse to flourish — which means that producers of natural wine, who tend to add little or no sulfur to their wines, are being forced to reckon with mouse in a very real way.

That’s the especially fascinating thing about mouse right now. Natural wine has long preached an acceptance of cosmetic blemishes: Where a typical California winery will go to great lengths to avoid so-called faults like volatile acidity, brettanomyces or reduction, many natural winemakers embrace these phenomena (at least in small volumes) as part of a living wine’s intrinsic, warts-and-all beauty. But I have yet to encounter a winemaker, natural or otherwise, who would accept the presence of mouse.

Will this become the natural-wine world’s first universal zero-tolerance policy?

“It’s in a different category,” says natural wine expert Alice Feiring, author of the new book “Natural Wine for the People.” “Mouse is enough to really destroy a wine.”

The phenomenon of mouse is not new, nor is the terminology. The first known documentation dates to 1894, in J.L.W. Thudichum’s “A Treatise on Wines.” It’s been observed in California wines since at least 1936. Feiring first began perceiving mouse around 2002 or 2003. “I didn’t know what it was called,” she says. In her first book, published in 2008, she called it “puppy breath.” Most of the time, it was subtly unpleasant, “like the wine just wasn’t ultra-focused,” she says, “but then every once in a while I’d come across something truly horrible.”

But over the last five or six years, Feiring has watched that occasional horror become much more frequent. Why? The fact that so many more people are making wine with extremely low levels of sulfur, or entirely sans soufre, is certainly part of it. Sulfur, a preservative, is known to eliminate the chemical compounds that can cause mouse. (Sulfur is also thought to remove some of a wine’s innate personality, which is why natural wine producers eschew it.)

On top of that, the growing demand for natural wine, and specifically for young, easy-drinking, light-bodied natural wines, means that wineries are bottling much earlier than in the past. “Before, you really made sure the wine was stable before releasing it,” says Feiring. Now, some wineries might release 2019 whites and rosés as early as January 2020. She believes that some of the mouse that’s making its way into bottles might have gone away if the winery had waited another six or 12 months before bottling it.

Climate change, too, may be aiding mouse’s ascendance. Warmer temperatures tend to produce higher pH levels in wine, and this seems to be a factor in the development of mouse. “Something about climate change is introducing something new into our environments,” says Brianne Day, owner-winemaker at Day Wines in Oregon. “Strains of bacteria are living in vineyards that didn’t used to.” She’s noticed chemistries in some of her wines that would have been unheard of a few years ago.

“Everyone agrees on what it is, but no one really gets it,” says Alex Pomerantz, winemaker at Subject to Change Wine Co. in Forestville. “There’s this playbook on here’s how you avoid it, here’s what causes it, and all those things make sense in the textbook, but in practice I have found exceptions to all of it.”

What’s clear is that three chemical compounds can be involved: 2-ethyltetrahydropyridine (ETHP), 2-acetyltetrahydopyridine (ATHP) and 2-acetylpyrroline (APY), all of which can have a roasted or cracker-like flavor on their own. ATHP is found in taco shells, corn tortilla chips, popcorn and rice cakes; APY is a flavor in basmati rice and pandan leaves (and has “tentatively” been identified in tiger urine, according to a 1990 paper in the journal Nature).

“It can taste like cashews or peanut butter,” says Lisa Costa, co-owner of Oakland natural wine bar the Punchdown. “Or sometimes it can taste more like rising bread dough.”

“I think it could be described as a whole litany of flavors,” says Pomerantz. It can taste like cereal milk, or it really can recall a rodent cage, he says. “Of course, most people have never actually tasted a mouse, but when it’s really bad it actually does kind of feel like there’s a jittery little weird mouse in my mouth.”

But the conditions that allow ETHP, ATHP and APY to take hold are not necessarily predictable. The presence of lactic acid bacteria and the yeast brettanomyces are strongly associated with these compounds, but then not every wine with brettanomyces or lactic acid bacteria develops mouse. The fact that a wine that has mouse may also have the funky barnyard flavor of brett may make it even more difficult for a novice wine drinker to identify it.

To make matters even more confusing, mouse cannot be smelled. The sensation is activated by your own mouth, whose higher pH renders the compounds volatile. (Since the perception of mouse depends on an individual’s saliva, it’s conceivable that everyone perceives it differently. Researchers agree that people are genetically predisposed to taste it or not.)

“The easy way to detect it — the hard way being tasting it — is to run a bit of the wine between your palms and then smell it,” says Linda Bisson, UC Davis professor of viticulture and enology. Your hand’s pH “will make the compound more volatile and therefore detectable.”

Is there such a thing as tolerable mousiness? To some palates, yes. Pomerantz describes some wines as having “mouse around the corner”: It’s there, but doesn’t meaningfully interfere with your drinking experience. “But then sometimes the mouse is in the house,” he says. “And that’s when you’ve gotta dump.”

“I’ve had plenty of wines that had some underlying mousiness that I thought were fantastic,” says Aran Healy, owner of the shop Ruby Wine in Potrero Hill. For the most part, he dislikes mousy wines, but “I don’t love the idea of labeling a wine as flawed,” Healy says.

This gets to one of the existential questions at the heart of the natural wine ethos. Is it really flawed, or is it just not to his individual taste?

“At Ruby we’ve developed a mantra,” he continues: “Serve it cold and drink it fast.” Whatever you do, do not decant. Since oxidation can exacerbate mouse, it will likely intensify as a bottle sits open. If he’s going to sell a customer a mousy wine — which he tries not to — he makes sure she understands the serve-cold-drink-fast mantra.

Pomerantz, who does not add sulfur to any wines, made a rosé in 2018 that developed mouse after he bottled it. “It was a bell curve — it got better and then worse again,” he says. He decided not to sell the wine. But a few months later, the mouse had mysteriously resolved, so he sent the wine out into the world. Now, you’d never know a mouse had ever been there. Like Feiring, he believes the best prescription for a mousy wine isn’t necessarily sulfur — it’s simply time.

My own experience confirms the proliferation: Suddenly, everywhere I go, I’m tasting mouse. Is it because of climate change and sloppy winemaking and early bottlings? Probably. Is it because I’m more attuned to it than I used to be? Yes. Like many wine drinkers, I was tasting it for years without really knowing what it was. Is it because there used to be only a couple of places in the Bay Area where you could find natural wines, and suddenly there are dozens, and I’m encountering more low- or no-sulfur wines than I ever was before? That, too.

I dislike mousy wines. If I order a wine in which I perceive a lot of mouse, I will send it back. I hope that the growing awareness of mouse will compel researchers to try to understand it better, and winemakers to work thoughtfully to avoid it. But my greater fear is that natural wine’s detractors will capitalize on the rise of mouse to argue that natural wine is a necessarily bankrupt enterprise. That’s unsound logic. These are early days in our understanding of this strange vinous rodent, and the most idealistic of our winemakers deserve a little leeway as they learn how to consistently navigate around it.

“Would you rather not have mouse in the house? Of course,” Pomerantz says. “But sometimes the mouse is two blocks away, and I’m like, give the wine a break. Dude, it’s natty wine.”

Esther Mobley is The San Francisco Chronicle’s wine critic. Email: emobley@sfchronicle.com Twitter: @Esther_mobley Instagram: @esthermob