Epilepsy
Seizures are a misfiring of neurons in the brain. A person is considered to have epilepsy when they have had two or more of these seizures. Epilepsy has many possible causes. It can develop because of an abnormality in brain wiring or an imbalance of certain neurotransmitters, or some combination of the two. This can be caused by a stroke, head trauma, tumor, or infection, but in some cases it may be idiopathic - with no known cause. There is some evidence to suggest in those cases there may be a genetic predisposition for epilepsy.
Epilepsy can be diagnosed with EEGs and brain scans that detect the abnormalities in the brain activity during a seizure. In order for a diagnosis to be made, though, there need to be recurrent seizures that are unprovoked, meaning they are not triggered by any obviously harmful thing, and are triggered by normal everyday things such as sleeping, illness, stress, alcohol consumption, or the most well known, flickering lights (which is only a trigger in about 5% of people with epilepsy.)
There are different types of seizures, classified first based on the source and second based on how much it affects consciousness. By source, there are partial (focal) seizures, which only involve a localized part of the brain, and generalized seizures, which involve all of both hemispheres, resulting in a loss of consciousness. Secondary generalization is when a partial seizure spreads to the whole brain and becomes generalized. Under partial seizures, there are simple partial seizures where consciousness is not impaired, and complex partial seizures, where it is impaired. Under generalized seizures there are 6 types: absence seizures (where the person becomes vacant for a short time), also known as petit mal, myoclonic seizures (which involve a brief muscle contraction less than 1/10 of a second), clonic seizures (regularly repeating muscle contractions at about 2-3 per second), tonic seizures (continuous...
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