Most adults have had a headache at one time in their life. In fact, according to the World Health Organization (WHO), up to three-quarters of adults in the world have had a headache in the last year. What you may not know is that there are 150 different kinds of headaches. Headaches are either primary or secondary. Headaches that are secondary mean they are caused by another condition, such as hormonal imbalances, a head injury, dehydration, or caffeine withdrawal.