Anxiety is the body's way of responding to being in danger.

Adrenaline is rushed into our bloodstream to enable us to "fight" or "flight". This happens whether the danger is real or not. This is the body's alarm mechanism and primitive man wouldn't have survived for long without this life-saving response. It works so well that it often kicks in when it's not needed; when the danger is in our heads rather than in reality. Thinking we're in danger is enough to trigger the system.

People who get anxious tend to go into "scanning mode", a mode where they're constantly on the lookout for danger, hyper-alert to any of the signals, and make it more likely that the alarm system will be activated.

There are different types of anxiety; an example might be Health Anxiety:

"I am always at the doctor's surgery. That is because I often worry about having something wrong with me. Last week it was tingling in my hands and arms. I thought it was the first sign of something like M.S. The doctor did some tests and said it was nothing to worry about. At first I felt better, but this week I have had a headache too, maybe I should go back just to make sure..."

This is an example thought of someone who suffers from health anxiety. That means that they worry a lot about their health. This focus on health makes their symptoms seem worse. Much of their time is taken up with these worries and reassurance from a doctor that nothing is wrong doesn't seem to help for long. If you find yourself with many troublesome worries about your health you may be experiencing health anxiety.

It might be helpful to identify if one of below definitions fits for you:

Health Anxiety Constantly worrying about having a serious illness.
Generalised Anxiety Disorder Constant worry about everything: "what if...?".
Obsessive Compulsive Disorder Intrusive thoughts mean I must check or do rituals.
Panic Disorder Repeated panic attacks.
Phobias Fear of particular situations.
Post Traumatic Stress Disorder Flashbacks, anxiety and anger relating to trauma.
Social Anxiety Anxiety in social situations; fear of being criticised by others.