Computer Soldier Porygon" (although most commonly known as "Electric Soldier Porygon") is the thirty-eighth episode of the Pokémon anime's first season.
In the episode, Ash and his friends find at the local Pokémon Center that there is something wrong with the Poké Ball transmitting device. To find out what's wrong, they must go inside the machine.
Twenty minutes into the episode, Pikachu stops vaccine missiles with its Thunderbolt attack, resulting in a huge explosion that flashes red and blue lights.
That was the cause of the seizures Japanese Children had.
It was tragic, and many children had to go to the hospital for medical care. Japan's Fire Defense Agency reported that a total of 685 viewers – 310 boys and 375 girls – were taken to hospitals by ambulances.