You see these sticks with a sponge they are holding or dunkin' in on the left side?
Instead of toilet paper, you may have used a sponge (Latin: tersorium) to wipe. These ancient devices consisted of a stick with a vinegar- or salt water-soaked sponge attached.
Parasite loads found in the end-location of Roman latrines are often... significant.
There's ongoing debate as to whether Roman urban hygeine was better, worse, or net neutral compared to rural (including rural 'barbarian') practices. About the only thing that's agreed is that it was better than non-Roman urban hygeine, which often made sharing a poop stick seem positively sanitiary.
Though, as noted, they were rinsed with water and soaked in either salt water or vinegar (which the Romans knew 'cleaned' better, though they didn't have a strong conception of bacteria or microscopic parasites)
I wonder how historians find out about this. Do we entirely rely on surviving written records? Or is there any other way to know what a seeningly poop stick was and how hygienic latrines were?
There are various written sources, including graffiti and poetry, for the use of the tersorium (poop stick), including one mosaic in an ancient latrine reminding users to "Make use of the tersorium" before leaving.
As for finding out about hygiene, ancient poop, believe it or not, is often an important archeological find because of the details we can get from it. Ain't science grand? There are other matters too, of course - Romans wrote effusively on their bathhouses, many of which survive to this day, as well as poetry admonishing men to groom themselves properly, physician's manuals, finding nit combs, etc etc.
Academics disagree as to its exact use, about which the primary sources are vague. It has traditionally been assumed to be a type of shared anal hygiene utensil used to wipe after defecating, and the sponge cleaned in vinegar or water (sometimes salt water).Other recent research suggests it was most likely a toilet brush.
Why would they? Your butt is really well designed for keeping stuff out, it's very hard to get infection or parasites that way (assuming you're not sticking it up there).