Yes, Diarrhea Can Be Sexually Transmitted. It Is Called Shigella, and We Read the Numbers Behind This Week’s Headlines

For most of a century, Shigella was a story about daycare centers, cruise ships and tap water: a bacterium that lives in stool, needs only a small amount of it to start an infection, and travels wherever hands, food and water carry it. Somewhere in the last two decades it found a shorter route between people. This week the internet learned the phrase for that route, sexually transmitted diarrhea, and reacted the way the internet reacts. We did the other thing. We read the two surveillance reports the headlines are built on, the CDC’s April analysis of 16,788 American Shigella isolates and the UK Health Security Agency’s March count of every sexually transmitted case in England, and set the numbers down in order, without the exclamation marks.

The bug: What Shigella is, in the CDC’s words

Shigella is a family of four bacterial species; Shigella sonnei is the common one in the United States, Shigella flexneri the second. The CDC’s plain-language page gives the course of the illness in three numbers: symptoms usually start one to two days after infection, last about seven days, and “people who have shigellosis usually get better without antibiotic treatment in 5 to 7 days.” The symptoms are diarrhea that can be bloody or prolonged, fever, stomach pain and the persistent urge to go when nothing is left. The agency’s clinical page puts the national toll at about 450,000 infections a year, 242,000 of them resistant to at least one of the usual antibiotics.

How it moves is the whole story. Shigella spreads, the CDC says, when people “put something in their mouths or swallow something that has come into contact with stool of someone infected.” Contaminated food and water do that. So do unwashed hands. And so, the same page adds, can “sexual activity with a sick person.” The germs are in the stool of a person with diarrhea and, the CDC’s page for sexually active people notes, “can continue to be found in their poop for up to 2 weeks after the diarrhea has gone away.”

The phrase: Is it really an STI?

Not in the way gonorrhea or syphilis are. Those bacteria need sexual contact; Shigella needs a mouth and a trace of stool, and sex is one of several ways to arrange that, with oral-anal contact the most direct. That is why the public health agencies say “sexually transmitted” rather than “sexually transmitted infection,” and why the same bacterium still turns up in the same places norovirus does. What has changed is which route is carrying the resistant strains. The CDC lists gay, bisexual and other men who have sex with men among the groups more likely to be infected, and states plainly that they are “more likely than others to be infected by antimicrobial-resistant strains.”

The American numbers: From none to 8.5 percent in twelve years

The CDC’s report, published in the Morbidity and Mortality Weekly Report on April 9, 2026 by a team led by epidemiologist Naeemah Logan, is an analysis of every Shigella isolate with resistance data that reached the agency’s PulseNet surveillance network from January 2011 to October 20, 2023: 16,788 of them. It uses a strict definition. An isolate counts as extensively drug-resistant, XDR, only if it resists all five antibiotics on the recommended list: ampicillin, azithromycin, ceftriaxone, ciprofloxacin and trimethoprim-sulfamethoxazole. By that bar, 510 isolates qualified over the whole period, 3 percent. The trend inside that figure is the finding: XDR was 0 percent of isolates from 2011 through 2015 and 8.5 percent in 2023.

The report also describes who those 510 people were. The median age was 41. Of the 492 with complete records, 96 percent were adults and 86 percent were men. Among the 116 whose HIV status was recorded, 47 percent had HIV. Among the 258 with a known outcome, 38 percent were hospitalized; no one died. Two thirds of the isolates were S. sonnei, the rest S. flexneri, and the flexneri share was highest in the West and Northeast. Most patients reported no travel. The sentence the clinicians underline is this one: for XDR strains, “no Food and Drug Administration–approved oral antimicrobial agents are available.” Logan told CIDRAP the pattern was “very concerning.”

The English numbers: Three years, three rises

England counts this differently and, in one respect, better: since 2016 the UK Health Security Agency has tracked Shigella diagnoses in adult men with no travel history as a proxy for sexual transmission, and publishes the series every spring. The March 26, 2026 report gives 2,052 such diagnoses in 2023, 2,318 in 2024 and 2,560 in 2025, more than half of them in London. The resistance figures are the ones that traveled this week. Of 416 S. sonnei samples sequenced in 2025, 86 percent were resistant to multiple antibiotics and more than half met the UK’s XDR definition, which is resistance to the three drugs British doctors reach for: ciprofloxacin, azithromycin and ceftriaxone. Of 490 S. flexneri samples, 94 percent were resistant to multiple drugs and 9 percent were XDR. Because the British bar is three drugs and the American bar is five, “more than half” and “8.5 percent” are not the same measurement, and neither country’s figure can be read as the other’s.

“The rise in sexually transmitted Shigella cases is concerning,” Katy Sinka, who heads the agency’s STI section, said in its statement, “but the risks of catching it can be reduced by good hygiene during and after sex and condom use.”

The genomes: Why the sexual route runs faster

The reason the resistant strains ride this particular route was worked out in July, in a Lancet Infectious Diseases study from the University of Cambridge and UKHSA led by Julia Marshall and Kate Baker. The team sequenced 3,514 S. sonnei genomes collected across the UK between 2004 and 2020, a third of them from men presumed to have been infected through sex, and used the genetic relationships between them to measure how each kind of strain moved. Strains in sexual networks had traveled a mean of 117 kilometers between related cases, against 46 kilometers for other domestic strains. They were 1.15 times as fit as strains brought back by travelers, and the lineages resistant to azithromycin grew 1.71 times faster than their susceptible relatives. The university’s summary adds the clinical cost: up to a third of patients admitted to hospital, for an average of four to five days. Baker’s description, in interviews, is that the disease has gone from relatively treatable to close to untreatable.

One mechanism is worth knowing because it has nothing to do with Shigella. In an expert reaction to the study for Spain’s Science Media Centre, the microbiologist Alicia Calvo-Villamañán described “bystander resistance”: an antibiotic given for one infection selects for resistance in a second bacterium the patient happens to carry. Azithromycin prescribed for gonorrhea can do that to Shigella, and she noted that when UK guidelines stopped recommending azithromycin for gonorrhea, the rise of azithromycin resistance in Shigella slowed. The Lancet authors make the same point in their last line: stewardship has to work across pathogens, not one at a time.

Treatment: What resistant means at the bedside

For most people it changes nothing, because most shigellosis is not treated with antibiotics at all; the CDC reserves them for select cases and asks clinicians to “select an antimicrobial agent based on the susceptibility profile of the individual isolate.” The resistance matters for the minority who need treatment: people with severe illness, people whose immune systems are compromised, and the roughly four in ten XDR patients in the CDC series who ended up in hospital. When the oral options are gone, what is left is intravenous. A UCLA case report of two Los Angeles patients carrying an XDR S. sonnei strain new to the literature, a man with advanced HIV and a woman with lymphoma living 80 miles apart, records that both needed intravenous carbapenems, a drug class hospitals hold back for last.

The advice: What the two agencies tell people, in their words

We are not a clinic, so here is what the agencies say. The CDC’s page for sexually active people asks people to wash hands, genitals and anus with soap and water before and after sex; to use barriers such as condoms, dental dams or cut-open non-lubricated condoms during oral and oral-anal sex; and, if a partner has bloody or prolonged diarrhea, to avoid sex and “wait at least 2 weeks after diarrhea ends.” Its general page adds the household rules: no cooking for others while sick, no swimming, no sharing towels. UKHSA’s version is seven days after the last symptom rather than fourteen, plus a full sexual health screen for anyone diagnosed, because, in Sinka’s words, they “may also have been exposed to other sexually transmitted infections, including HIV.” Both agencies say the same thing about the diarrhea itself: if it is bloody or lasts more than three days, it belongs in front of a clinician, with a stool test, which is also what we say about blood in stool generally.

The facts to carry out of this week: the bacterium is Shigella; it spreads through traces of stool, by sex among other routes; in the United States the share of isolates resisting all five recommended antibiotics went from zero to 8.5 percent between 2011 and 2023; in England, sexually transmitted diagnoses rose three years running to 2,560; most people recover in a week without antibiotics; the ones who need them are the problem. The article that started this week’s wave got the numbers right. It is the word “exploding” we left out. The CDC, UKHSA, the University of Cambridge and UCLA had no involvement in this article. It is educational, describes what the cited reports and studies say, and is not medical advice.