A stroke is a reduction in blood supply to the brain due to a blockage (80% of strokes), which is usually a blood clot, or bleeding in the brain (20% of strokes). Rapid treatment—within three hours of the first symptom—is crucial. If blood supply is quickly restored, it may prevent permanent brain damage. Stroke symptoms come on suddenly and include trouble speaking; confusion; weakness, numbness, or paralysis on one side of the face or body; vision problems; severe headache; or trouble with walking or balance. Call 911 if you or someone else has these symptoms.