Both positive and negative effects decay. I'm not sure, but perhaps it's a bad idea to build -crime/disease buildings or units (especially buildings) when you don't need them, it seems like their benefits dissipate to nothing and then later, in a crisis, they're no help. In my experience education changes per turn are a lot more stable, eg. if you have a -5/turn, education will keep going down until you do something about it. At -300 crime or disease, I would only worry about +41 or more per turn. I find it helpful to multiply the per turn by 10, to find out what the current change would result in if it persisted for 10 turns (it certainly won't, and it's more likely to move towards zero than the other way, but I find it a good rule of thumb that avoids too much harmful thinking...). The main other factors I can think of are: population increase makes crime and disease jump up (and education jump down), and then if there are criminals in your city (including your own), they mostly have a huge impact (conversely getting rid of them/moving them away gives a big crime per turn reduction).