"Death is bad," said Harry, discarding wisdom for the sake of clear communication. "Very bad. Extremely bad. Being scared of death is like being scared of a great big monster with poisonous fangs. It actually makes a great deal of sense, and does not, in fact, indicate that you have a psychological problem."
...
But some people fear death all the time - they're sitting at home, watching tv, and they can feel their heart beating, and they think it's beating in a weird way, and that causes them to feel fear, which causes a bunch of physical reactions. Tense muscles, sweating, breathing changes; the person becomes aware of these and instead of ascribing them to fear they think these are the start of a heart attack. This causes even more fear and anxiety. Their heart is now hammering. They are "hyper aware" of their heart beat and breathing and sweating. They may well feel pain. This is not "in their head". It is real actual pain. It is the same pain that you would feel if someone punched you in the arm. They call an ambulance. They are taken to hospital.
In that situation no doctor is going to say "Don't be stupid, you're not having a heart attack" (because people do have heart attacks), so the doctor will strap the person to all sorts of machines.
But this doesn't relieve the anxiety. If anything, it just makes things worse. "They wouldn't spend all this money and time if it was just anxiety! I wouldn't be in hospital for this long if there was nothing wrong!"
And, when all the results come back negative that doesn't relieve anxiety either. "Oh God! It must be really serious, because they can't find it on their tests! Maybe it's cancer!"
Anxiety disorders quickly develop a strong grip on sufferers. Behaviours can be extraordinary. You only have to see the rituals that people with OCD go through to realise this.