The band was founded in Cracow, in 1999. The same year their first demo, Art of Decadence, appeared. It consisted of four tracks plus a bonus - cover version of Madonna’s "Frozen" (performed with Anna Wojtkowiak from Centuria). As the demo was very warmly welcomed on the underground scene the band decided to continue their musical activity.
Also in 1999 Thy Disease won the second prize at Metal Millenium Party and Vader invited them to play as a support act on their Polish tour.