The Lavender Town Syndrome (also known as "Lavender Town Tone" or "Lavender Town Suicides") was a peak in suicides and illness of children between the ages of 7-12 shortly after the release of Pokémon Red and Green in Japan, back in February 27, 1996.
Rumors say that these suicides and illness only occurred after the children playing the game reached Lavender Town, whose theme music had extremely high frequencies, that studies showed that only children and young teens can hear, since their ears are not fully developed. -Wikja
Basically it was a high pitched sound that kids without fully developed ears could hear, and many of them committed suicide because of it. The ones who didn't complained about headaches and such.