Stress

Stress is the short term effect of seeing zombies nearby, hearing banging at your door, hearing unsettling noises, and generally anything which upsets the character (which will likely include annoying companions in the future). If nothing is bothering the character, his stress level will begin to drop. However, while the character is stressed his morale will decrease, which is a more long term effect and harder to change.

Stress can be lowered by killing zombies (ultimately this would depend on the personality of the player), sleeping, drinking whiskey, taking painkillers, or smoking cigarettes.