https://www.newscientist.com/article/mg23130800-600-ancient-rome-the-empire-that-cleaned-up/
The Romans brought their famous baths and toilets to three continents, so how come they left places more unhygienic than they found them?
There is a discussion on Hacker News, but feel free to comment here as well.
Link to the research. From the research itself, it seems mostly three sanitation issues:
The first two issues would be relatively easy to solve, if they knew what caused the problem. The third one is trickier: liquamen requires uncooked fish because it relies on fish digestive enzymes dissolving its flesh, and those enzymes denature under heat. There are a bunch of solutions but all of them would change its taste, such as cooking the fish and then innoculating it with qū/kōji (unknown for the Romans, but roughly from the same time period as the Greeks started producing their garos), or preparing it in the traditional method and then boiling it between fermentation and distribution (once parasite-free, liquamen is pretty much safe - it's too sour and salty for bacteria and funghi).