Toilets have a long history. The aim of this article is to examine the influence of water and toilets on public health during the Roman era (circa 200 B.C.–500 A.D.). Toilets during the Roman era can be divided into two groups: public and private. A public toilet was often built in proximity to or inside a bath so that it was easily entered from both inside and outside of the bath. The abundance of water that was conducted to the bath could also be used to flush the toilet. Piped water for flushing private toilets seems to have been a rarity. In many cases the private toilet was located near the kitchen. Hygienic conditions in both types of toilets must have been very poor, and consequently intestinal diseases have been all around. Dysentery, typhoid fever and different kinds of diarrhoeas are likely candidates for diagnoses. Descriptions of the intestinal diseases in the ancient texts are unfortunately so unspecific that the identification of causative agents is a very problematic venture. Studies of ancient microbial DNA might offer some new evidence for the identification of microbes spread by contaminated water.