A stroke occurs when a blood vessel to the brain is blocked by a clot, preventing the brain from receiving oxygen-rich blood. This can cause brain cells to die. Signs of a stroke include face drooping, arm weakness, speech difficulties, and severe headaches - it is a medical emergency requiring calling 911 immediately. Risk factors like high blood pressure, smoking, diabetes, and high cholesterol can be controlled through healthy lifestyle changes such as diet, exercise, and regular doctor visits to help prevent strokes from occurring.