The Epilepsy Foundation had to shut down the forums that it oversees due to people posting malicious images.
These flashing images were used to trigger seizures in people visiting the site over the Easter holiday.
The story first appeared in Wired News, as vandals took advantage of the low level of security for the forums to insert a Trojan horse that would constantly flash an image.
This flashing image were responsible for triggering epileptic seizures in some forum members.
Since then security has been tightened to prevent further attacks.