A stroke occurs when there is a sudden loss of brain function due to lack of oxygen to the brain. The lack of oxygen is due to loss of blood flow caused by either an important artery in the brain being closed by a clot or by tearing apart and blood flows elsewhere. One of the important warning signs of a stroke is a severe and unusual headache that is sometimes described as "the worst headache ever."
Some risk factors for stroke can be controlled, such as smoking, while others are beyond our influence, such as genetics and age. The effects of a stroke depend on which part of the brain was affected, and how severe the damage was.
Stroke is classified as an emergency situation because the damage of a stroke can be lessened by a quick response. Depending on the extent of impairment, rehabilitation and recovery are also possible.