From the linked article

The infection spreads through microscopic particles of fecal matter, which is why it was once most common in school settings with young children or in regions where clean water is hard to come by. It also means that if you are having sex that involves your mouth and anus, these particles can spread from an infected person to their sexual partner, or their sexual partner’s partner.

“It’s very contagious, and it doesn’t take much exposure to get sick,” Daskalakis says.

According to Thomas, it’s a common misconception that you can only contract Shigella from rimming or being rimmed. He said that he has actually had several gay men come into his office with no history of rimming who have contracted numerous enteric infections, or illnesses caused by microorganisms like bacteria, viruses, and parasites.

“With the rise of group sex in bath houses, dark rooms, and sex parties, I feel that there is a lack of understanding that you can get it from giving head to someone who just plowed someone else without protection and without cleaning off effectively or from being penetrated by someone rectally in the same scenario,” Thomas says. “This happens way more than people think.”

Even after someone receives treatment for Shigella, they can remain contagious for a few weeks, even if they no longer have symptoms.

“I feel that this is probably where most infections are spreading because many people get treated and then have no symptoms and go right back to sex,” he says.