If it is spies causing you trouble, you can see from mousing over the health icons in the city screen. You can also use Ctrl+Tab to see the turn log which will tell you if you got poisoned without knowing it. It is good to pay attention to the turn log ... usually, a big city is either working some mines or specialists and can add enough food to make it through the poisoning (which is -8 health (not sure if level specific) that decays each turn by 1) So you only lose 36 food from having water poisoned (assuming you were at or above health cap). Usually if you have a granary, and haven't already starved most of your food away, poisoning doesn't hurt that much.
Since you talk about going from 25-15, I imagine the problem is war weariness. In addition to the culture slider and whipping, you can control war weariness by making your wars shorter, winning more battles, and not going up against the Statue of Zeus (or taking it immediately). War weariness decays at about 1% per turn, and has no effect unless actually at war with that AI. So if you have a lot of WW built up against one AI, and plan a war against another, try not to be at war with both at the same time. Build jails and Mt. Rushmore, and if it gets bad enough, change civics to police state.
If your water supply is getting poisoned, put a spy in each city ... it helps catch the spies. Also you can build Security Bureaus and run counter espionage missions to make it more difficult for the enemy to run spy missions.