Seizures are a symptom of epilepsy. Epilepsy is the underlying tendency of the brain to produce sudden bursts of electrical energy that disrupt other brain functions. Having a single seizure does not necessarily mean a person has epilepsy. High fever, severe head injury, lack of oxygen–a number of factors can affect the brain enough to cause a single seizure. Epilepsy, on the other hand, is an underlying condition (or permanent brain injury) that affects the delicate systems which govern how electrical energy behaves in the brain, making it susceptible to recurring seizures.