Antibiotic resistance is once again making headlines—this time not involving an STD, but a brutal GI illness that can also spread sexually.

In January, U.S. public health officials warned of a so-called “super strain” of gonorrhea resistant to many types of antibiotics usually used to treat it. A little over a month later, the U.S. Centers for Disease Control and Prevention is raising red flags regarding a strain of a diarrhea-causing bacteria with similar antibiotic-evading properties.

Cases of XDR, or “extensively drug resistant,” Shigella, are on the rise in the U.S., the CDC said in a Feb. 24 alert to clinicians. In the U.S., all cases of Shigella must be reported to public health authorities. In 2015, no reported cases were drug resistant. In 2022, 5% were.

Colorado has seen one of the largest rises in cases of XDR Shigella in the nation, likely due, at least in part, to the state’s ability to do high-volume testing, Rachel Jervis, an epidemiologist with the state’s health department, tells Fortune.

She calls the rise of drug-resistant Shigella, and other pathogens, “concerning.”

It’s never been more important for clinicians to ensure that the antibiotics they’re prescribing are appropriate, she says—and for patients to understand that if their provider isn’t prescribing antibiotics, it may be for good reason. That’s because pathogens have the ability to evolve to evade treatments. The more frequently such treatments are used, the greater the chance that such evolution occurs.

Here’s what you need to know about this potentially tough-to-treat GI illness rearing its ugly head in the U.S. and abroad.

What is Shigella?

Shigella is a type of bacteria that causes shigellosis, an illness involving inflammatory diarrhea that is often bloody and may be prolonged. It causes just shy of a half million infections a year in the U.S., resulting in nearly $100 million in medical costs, according to the U.S. Centers for Disease Control and Prevention.

What are the symptoms?

Aside from diarrhea, which may last for more than three days, symptoms can include:

 Fever

 Stomach pain

 Feeling the need to defecate even when your bowels are empty

What is “XDR”‘XDR’ Shigella?

XDR is short for “extensively drug-resistant.” Most Shigella infections don’t require antibiotics, Jervis tells Fortune. But some, like those in people who are immunocompromised, do require antibiotics. They can also be used to shorten an infection and reduce the chance of an infected person spreading it to others.

In the case of XDR Shigella, fewer antibiotics work to cure the infection. According to the Feb. 24 CDC alert, XDR Shigella is resistant to all commonly recommended and alternative antibiotics. There isn’t enough data to make a recommendation of other antibiotics for these infections, the CDC says.

How does Shigella spread?

Like norovirus and polio, Shigella spreads via fecal-oral transmission—and it takes only a very small (microscopic, actually) amount to make someone sick.

Shigella usually spreads among close contacts. The bacteria can get into your mouth if you touch surfaces like toys, bathroom fixtures, diaper pails, and changing tables contaminated with feces that contain the bacteria, and then your mouth, according to the CDC. (The feces on contaminated surfaces may not be visible.)

You could also become infected by:

 Caring for someone who is infected, including cleaning up after the person uses the bathroom

 Eating food made by someone who is infected

 Swallowing water in a lake or improperly treated swimming pool

 Consuming contaminated drinking water

 Being exposed to feces during sexual contact with someone who is infected or who has recently recovered from an infection

Who is at greater risk for a Shigella infection?

Shigella is most common in young children, according to the CDC, due to lack of proper hand hygiene. Many cases occur in daycares and schools. Travelers are also at risk, the national public health agency adds.

Aside from people with weakened immune systems, the other major risk group is men who have sex with men, the CDC says. Shigella can pass from feces or soiled fingers to another person’s mouth during sexual activity.

The CDC cautions all patients who’ve had Shigella to wait to have sex of any type—vaginal, anal, or oral—until two weeks after their diarrhea has stopped, and to use safe sex practices for several weeks after they resume sexual activity, as the bacteria can remain in the stool for weeks.

How can I prevent a Shigella infection?

The good news is that basic hygiene will go a long way. The CDC recommends:

 Carefully washing your hands with soap and water before sexual activity, eating or preparing food, and after going to the bathroom, changing a diaper, or cleaning up after someone who went to the bathroom

 Throwing away diapers in a covered, lined garbage can

 Cleaning up mess from diapers thoroughly and promptly

 Avoid swallowing water from lakes, ponds, and swimming pools

 Refraining from sex when you have diarrhea, and for two weeks after diarrhea resolves

You can keep others well by staying home when you’re sick, and by not returning to work or childcare if you’ve had Shigella until you’ve been cleared by your local public health department to do so, Jervis says.

Public health workers can connect those who are not allowed to work due to Shigella infection with community resources that can help during the financially challenging time, she adds.

What is antimicrobial resistance, anyway?

Antimicrobial resistance occurs when bacteria, viruses, fungi, and parasites evolve over time, becoming less responsive to medicines, making infections increasingly difficult, or impossible, to treat.

“We are watching this antibiotic era turn into a post-antibiotic era,” Dr. Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP), told Fortune in late January. “That is a scary proposition.”

Global efforts are underway to slow the increase of resistance, since it’s an impossible process to stop, he said. But little is being done in this regard, or to develop new treatments. A November 2021 statement by the WHO called the clinical pipeline of new antimicrobials “dry.”

It wasn’t so long ago that “children died all the time from bacterial infections that today we wouldn’t think twice about—they’re easy to treat,” Osterholm said.

“Our great-grandparents grew up in a pre-antibiotic era,” he said. “My generation, in large part, and my kids’ generation, grew up in an antibiotic era. Our grandkids and great-grandkids are clearly going to be living—at least in many areas of the world—in a post-antibiotic era.”

Antimicrobial resistance, like climate change and preparation for the next pandemic, are insidious issues, relatively easy to ignore—until they’re not, he cautioned.

“One of the challenges right now is envisioning this new world and doing something about it,” he said. “We talk about it, think about it, hold a couple of meetings about it. But in the end, what have we really done to change the course of the future?”