Anxiety can run in families. Or a person might develop anxiety after something terrible happens, like a car crash. Sometimes certain medical illnesses can cause feelings of anxiety. So can abusing alcohol or other drugs, like cocaine.

Another part of the explanation has to do with the different chemicals in the brain. How the chemicals in our brain's nerve cells are balanced can affect how a person feels and acts. One of these chemicals is serotonin. Serotonin is one of the brain chemicals that helps send information from one brain nerve cell to another. But for some people with anxiety, this brain chemical system doesn't always seem to work the way it should.

Also, some scientists think that a special area in the brain controls the fight or flight response. With anxiety, it's like having the fight or flight response stuck in the ON position - even when there is no real danger. That makes it hard to focus on everyday things.