Seizures are a tier 3 symptom for a standard plague and the Mad Cow Disease scenario. They cause random blackouts and fits that reduce the patient's ability to do things without the assistance of others.
They are the prerequisite to Inflammation, making them a tier 4 symptom as well.
In real life
A seizure, formally known as an epileptic seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with loss of consciousness (tonic-clonic seizure), to shaking movements involving only part of the body with variable levels of consciousness (focal seizure), to a subtle momentary loss of awareness (absence seizure). Most of the time these episodes last less than 2 minutes and it takes some time to return to normal. Loss of bladder control may occur.