When you go to your doctor’s office to have a headache treated, many times you will hear him or her use the word "trigger" when referring to your headache. In the case of a headache, the word trigger means something that starts the headache—in other words it initiates it.
This does not mean that the triggers are direct causes of the disease, but they facilitate or provoke the beginning of an attack in a patient who already suffers from headache. For example, I have a patient who has the start of a headache attack every time she goes to a department store and walks by the cosmetics department. Her headaches are triggered by the perfume aroma.
