Epilepsy develops when something changes the way brain cells communicate, causing them to fire in abnormal, synchronized bursts. In roughly half of all cases, doctors can identify a specific cause. In the other half, no clear explanation is found. Around 50 million people worldwide live with epilepsy, and the causes span a wide range, from genetics to brain injuries to infections.
Genetics and Family History
Some people are born with gene changes that make their brain cells more likely to misfire. These aren’t always inherited from a parent. They can also appear as new, spontaneous mutations. In certain childhood epilepsy syndromes, researchers have identified single-gene mutations in roughly 5 to 16 percent of cases using modern genetic testing. Genes involved often affect how brain cells control their electrical signaling or how they grow and organize during development.
Genetic epilepsies don’t always follow a simple pattern. Sometimes a gene variant runs in a family but causes different seizure types in different relatives, or skips some family members entirely. A broad category called “idiopathic generalized epilepsies,” where seizures affect both sides of the brain from the start, accounts for 15 to 20 percent of all epilepsy cases. These are widely accepted to have a genetic basis even when the exact genes haven’t been pinpointed.
Traumatic Brain Injury
A serious blow to the head is one of the more well-understood paths to epilepsy. After a severe traumatic brain injury, 10 to 20 percent of survivors develop what’s called post-traumatic epilepsy. A large study tracking patients over time found the cumulative risk reached 25 percent at five years and 32 percent at fifteen years, meaning seizures can first appear long after the original injury has healed.
The timeline matters in a specific way. Seizures that happen within the first week after a head injury are considered “early” and don’t necessarily mean epilepsy will follow. Seizures that start more than a week after the trauma are a stronger warning sign. Among patients who had even one late seizure, the risk of having another was 62 percent within a year and 82 percent within ten years. The injury leaves scar tissue or altered circuits in the brain that can generate abnormal electrical activity indefinitely.
Stroke and Aging
Stroke is the single most common identified cause of new epilepsy in older adults. Overall, stroke accounts for about 11 percent of newly diagnosed epilepsy in adults, and that figure climbs higher in people over 60. When a stroke cuts off blood flow to part of the brain, the damaged tissue can become a source of abnormal electrical signals, sometimes months or years later.
Brain tumors and neurodegenerative diseases like Alzheimer’s can also trigger epilepsy in older adults by disrupting normal brain cell function. This is why epilepsy diagnosed later in life often prompts doctors to look for an underlying neurological condition.
Infections That Damage the Brain
Infections that reach the brain are a major cause of epilepsy globally, particularly in low- and middle-income countries where nearly 80 percent of people with epilepsy live. The most significant is neurocysticercosis, a parasitic infection caused by pork tapeworm larvae that lodge in brain tissue. In regions where this parasite is common, it accounts for roughly 30 percent of seizure disorders. The larvae form cysts that eventually calcify, leaving scars surrounded by inflammation. Seizures often persist for years after the parasite itself has died, because the scarring and tissue damage remain.
Bacterial meningitis, viral encephalitis (inflammation of the brain from infections like herpes simplex), and other central nervous system infections can all leave behind enough brain damage to cause ongoing seizures. The pattern is similar across these infections: the original illness damages or scars brain tissue, and that damaged area becomes an ongoing source of abnormal electrical activity.
Birth Complications and Early Development
The most common cause of seizures in newborns is oxygen deprivation during or around birth, a condition called hypoxic-ischemic encephalopathy. When a baby’s brain doesn’t receive enough oxygen, it can suffer widespread injury. The severity of seizures in these newborns closely tracks with the extent of brain damage visible on imaging. Some of these children go on to develop epilepsy that persists into childhood and beyond.
Structural brain abnormalities that form before birth also fall into this category. These include malformations where brain tissue didn’t develop or organize correctly during pregnancy. Some of these structural problems are themselves caused by genetic mutations, which is why the categories of epilepsy causes often overlap.
Metabolic and Immune Causes
Certain rare metabolic disorders directly cause epilepsy by disrupting brain chemistry. One example is pyridoxine-dependent epilepsy, where a genetic enzyme deficiency allows a toxic molecule to build up in the brain and interfere with vitamin B6 function. Since B6 is essential for producing the chemical messengers brain cells use to communicate, the disruption causes severe seizures, typically starting in infancy. This particular condition can be controlled with high-dose B6 supplementation, which is why identifying metabolic causes matters for treatment.
The immune system can also target the brain directly. In autoimmune epilepsy, the body’s immune defenses mistakenly attack proteins on the surface of brain cells, making them fire erratically. These cases are important to identify because they may respond to treatments that calm the immune system rather than standard seizure medications.
Triggers Are Not the Same as Causes
People sometimes confuse seizure triggers with the underlying cause of epilepsy. A trigger is something that lowers the threshold for a seizure in someone who already has epilepsy. Common triggers include sleep deprivation, stress, alcohol, flashing lights, and illness. These factors don’t create epilepsy. They make an existing tendency toward seizures more likely to produce one on a given day. Most people with epilepsy don’t have a single reliable trigger that causes a seizure every time.
When No Cause Is Found
Despite advances in brain imaging and genetic testing, roughly half of people diagnosed with epilepsy have no identifiable cause. This doesn’t mean nothing is wrong. It means current technology can’t yet detect the specific change responsible. Many of these cases likely involve subtle genetic factors, combinations of small-effect gene variants, or microscopic structural changes too small for standard imaging to pick up. As diagnostic tools improve, the proportion of “unknown” cases continues to shrink, but it remains the single largest category.