Humor is serious business. Sure, there's simple comedy like a pie in the face or an Adam Sandler movie, but a lot of jokes display real intellect, and despite much reflection and experimentation—both in labs and on stages—no one has yet discovered a unified theory of hilarity.
Stress generally refers to two things: the psychological perception of pressure, on the one hand, and the body's response to it, on the other, which involves multiple systems, from metabolism to muscles to memory.
Anxiety, or extreme apprehension and worry, is a normal reaction to stressful situations. But in some cases, it becomes excessive and can cause sufferers to dread everyday situations.
This type of steady, all-over anxiety is called Generalized Anxiety Disorder.