Cluster headache

Cluster headaches are a type of headache. They are the least common type of headache. 
Cluster headaches are very painful. They usually cause very strong pain on one side of the head, behind the eye and temple. The headaches can last anywhere from 15–180 minutes. People with severe cases may have cluster headaches many times a day. 
Cluster headaches may also cause other symptoms. Most people get restless or agitated during an attack. Some people get a runny or stuffy nose. The eye on the side of the head that hurts can also get red or start tearing up. 
They are called "cluster headaches" because they usually come and go. For a while, person may have many attacks of cluster headaches. Sometimes, they may then have no cluster headaches for a while. If a person has no cluster headaches for a while, this is called a remission period. No one knows why remission periods happen. Some people with cluster headaches never have a remission period. 
Cluster headaches can be genetic (passed down through families). However, when they are, a person may never have a cluster headache unless they have a bad head injury. 
In people who do not carry a gene for cluster headaches, nobody knows what causes them.