One of the lesser-known donts of Jewish law is not to eat meat and fish together. Religious people will avoid eating them at the same time, but will have them at the same meal, though usually with a break between them.
This rule appears in the section of the Shulchan Aruch about "items that are due to exposure". Drinks left uncovered are forbidden, lest a snake drink from them and leave venom behind. We are also advised not to put coins in our mouth in case they carry someone else's dry saliva.
In this context, another health and safety regulation was not to eat fish and meat together for fear it might cause a certain skin infection. Although this is more of a warning rather than a strict ban, the halachah does not take it lightly. "Danger is more serious than a prohibition", says the Talmud (Chullin 10a). Although we see no medical risk in eating fish and meat today, the prohibition survives as a sign of the seriousness with which Jewish law treated possible threats to health.