Link to the research. From the research itself, it seems mostly three sanitation issues:

• the water of the thermae wasn't changed frequently enough, so they were a vector for parasites to spread
• they used human faeces as manure for the fields, spreading parasites to the crops that would be eaten afterwards
• liquamen/garum (fermented fish sauce) widespread usage, responsible for fish tapeworms even outside coastal regions

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).