A new and unnerving Strep A threat is on the horizon: a version of the bacteria that’s partly or wholly resistant to penicillin and other antibiotics.

Streptococcus A is perhaps most infamous for being the most common cause of strep throat—a painful inflammation most common among children that can last weeks and cause serious complications if left untreated. The same strain is also responsible for a skin condition called impetigo. Although the CDC doesn’t track these “non-invasive” strep infections, it estimates that millions of them happen each year.

The bacteria is also behind some serious ailments, such as necrotizing fasciitis and acute rheumatic fever. Between 11 and 13,000 cases of these “invasive” strep A infections occur in the United States each year, the CDC estimates, with more than 1,000 deaths. But Strep is generally considered a known entity—with a known, reliable treatment.

Then came two serious Strep A infections in Seattle, which were described in a paper published last October. They represented a wake-up call for Strep A researchers, says James Musser, a Baylor College of Medicine pathologist at Houston Methodist, because they overturned long-held wisdom about the disease’s susceptibility to penicillin and other beta-lactam antibiotics. These antibiotics bind to enzymes that bacteria need for building their cell walls and make it impossible for them to keep cohesion.

The two Strep A infections weren’t totally resistant to beta-lactam antibiotics—that would have been front page news outside the microbiology community, as well as inside it. But they did display markedly more resistance to the drugs than normal. That was disturbing enough to inspire Musser and a team of international colleagues to start digging deeper. Their first results, published Wednesday in the Journal of Clinical Microbiology, show definitively that some strains of Strep A bacteria are less than 100-percent susceptible to beta-lactam antibiotics.

“It’s almost like a creep, very gradually occurring,” says University of Alberta clinical microbiologist Gregory Tyrrell, who was not involved with the study. What’s more, he says, the paper shows “that this is not a rare event.” Musser and colleagues didn’t find samples in their collection that were as antibiotic-resistant as the two Seattle specimens, but they still found indications that Strep A is slowly learning to fight back.

Now that there’s evidence the bacteria are mutating to be less susceptible to antibiotics, Musser says, the global health community needs to move fast to catch up. Decades ago, he says, researchers were saying they needed to understand strep better and develop a vaccine. But he and many of his colleagues “took our eye off the ball.”

After all, beta-lactams worked for every illness caused by Strep A. Now, there’s evidence they may be less effective for some, but we don’t even know which. “We have zero idea about what the extent of these mutations,” says Musser, “and whether decreased susceptibility to beta-lactam is in the most common form of infection.” That would be strep throat, known clinically as pharyngitis.

The bacteria that Musser and his team looked at came from a database of more than 7,000 samples from patients with invasive Strep A. They found that 2 percent of those samples had changes to a single amino acid that made them more resistant to antibiotics.

Musser says that small percentage is cause for major alarm. The simplicity of the changes suggests “it is potentially very easy for an organism to develop a decreased susceptibility,” he says.

“I think it warrants extra vigilance,” says Tyrrell.

When they’re exposed to antibiotics, bacteria have the opportunity to adapt and become resistant to their effects, with only the most resilient microbes surviving and reproducing from treatment to treatment. Slowly, some strains of bacteria have gathered more and more resistance traits to different kinds of antibiotics, which means they are much, much harder to kill. Mutations can even overlap to make superbugs capable of withstanding every drug we have.

A good way to decrease the amount of beta-lactam antibiotics that Strep A strains are exposed to would be to produce a vaccine, says Musser. Until recently, it hasn’t been a high priority on researchers’ radars, because it was thought that beta-lactam antibiotics worked against all forms of strep. However, several labs are working on developing a vaccine, and Tyrrell said that if Strep A was to become wholly beta-lactam resistant—the worst-case scenario—it would be possible to quickly develop a response.

However, he says the public shouldn’t be concerned at this point. For now, you can avoid getting Strep A and a host of other ailments by following the same old advice as always: washing your hands, avoiding touching your face, and practicing other basic hygiene measures.

Correction: A previous version of this article mistakenly referred to strep throat as a virus on one occasion, when it is in fact caused by bacteria. This has been corrected.