To save a trip to the South Park wiki, the season 5 episode is the infamous episode where Kenny dies “for good,” called “Kenny Dies.” In the episode, Kenny is hospitalized with something similar to muscular dystrophy, and his death is treated as a dramatic event, instead of a comedic one like in every episode before.
Trey Parker and Matt Stone, the show’s creators, had grown tired of killing Kenny, so they decided to kill him for good (although the death only lasted about one season). Originally, they had planned to kill Kyle off instead.
The reason was that they felt Kyle and Stan were too similar characters (as many have repeated in criticisms of the show). They wanted to kill off Kyle and bring Butters in as the new fourth boy. Butters had started to grow as a character and the creators started to become more interested in him.
The episode following Kenny Dies was one that focused mainly on Butters, called “Butters’ Very Own Episode.” Many South park viewers today still consider Butters to be the unofficial fifth boy of the group.