For most people, opening up Twitter or email doesn’t pose much risk beyond the danger of getting into a passionate argument over, say, politics or whether a hot dog is a sandwich. But people with epilepsy run the risk of coming across images or videos that could trigger seizures—or being trolled with such images when other users don’t like what they have to say. According to a thread of tweets from journalist Kurt Eichenwald, who has epilepsy, just such a situation happened this weekend. Now, he is bringing charges against the Twitter user who sent the tweet that triggered his seizure, the BBC reports.

“You deserve a seizure for your posts,” wrote the user, attaching an animated GIF with flashing, red-and-yellow images. It’s not the first time Eichenwald has been attacked—earlier this year, he received what he characterized as an “epileptogenic video” via email after penning a piece on Donald Trump’s business interests.

According to the Centers for Disease Control and Prevention, about 1.8 percent of American adults—about 4.3 million—have epilepsy, a brain disorder that causes seizures. A much smaller percentage of that group has experienced seizures triggered by lights or patterns. Called photic or pattern-sensitive seizures, these occur when people with epilepsy see a series of images or video that fall into a certain range of color, pattern and frequency.

In 2005, the Epilepsy Foundation of America convened a working group on these seizures and conducted a literature review to help better characterize epileptic photosensitivity. They found that people with epilepsy have a two- to 14-percent chance of having such a seizure.

Patients with epilepsy have reported having seizures from watching a wide range of things, the team found, from a spinning potter’s wheel to TV commercials that feature strobe-like lights. But they warned that photosensitivity and its relation to epilepsy is not very well understood. This is due in part to the fact that it’s not exactly ethical to gather patients with epilepsy and expose them to images that could trigger a seizure—especially since seizures can be deadly.

Researchers, however, can test photic seizures in animals, and they’ve done so on multiple occasions. A test on baboons suggested that, this type of seizure is linked to the cerebral cortex—the part of the brain that transmits visual information to the rest of the brain.

But the mechanisms by which seizures occur in humans are not yet fully understood. Right now, scientists think that one or multiple areas of the cortex can be stimulated by flashing or striped patterns, triggering abnormal, wave-like electrical activity inside the neurons of the cortex.

Scientists also know the characteristics of imagery that apparently triggers abnormal brain activity. Brightness appears to be the most intense trigger, followed by the rate at which an image flashes. Certain colors, like red, and patterns, like parallel lines and stripes, can induce seizures, too.

Even when an image isn’t intended as a weapon, it can inadvertently trigger epilepsy. Perhaps the most famous example occurred in 1997, when hundreds of Japanese children were taken to the hospital after suffering from seizures, vomiting blood, fainting or becoming dizzy while watching an episode of Pokémon. The culprit was a sequence during which red and blue lights flashed onscreen after a bomb exploded onscreen—which makes sense, given that red and blue stimuli have been tied to seizures for over a century.

The episode was banned and has never been shown on TV again, but scientists have been intrigued by it ever since. The so-called “Pokémon phenomenon” has been studied as an example of a visual epilepsy trigger ever since—though at least one study claimed that it was a case of mass hysteria instead. Since then, researchers have also spent time studying potential seizure triggers in things like video games.

As the Epilepsy Foundation writes, many people don’t even realize that they have epilepsy until a flashing light or image triggers a seizure. And though organizations like the World Wide Web Consortium, which sets international standards for the internet, have put guidelines in place for people who want to put flashing content on the web, compliance is voluntary.

Now that Eichenwald has gone public with his ordeal, he could help raise awareness of epilepsy triggers and increase research interest in the topic. But that increased attention could play into the hands of internet trolls intent on inflicting physical damage from afar. Perhaps lawsuits like Eichenwald’s will help discourage people from weaponizing flashing images—but until then, computer screens will remain a fraught place for people with epilepsy.