The Purple Smurfs is basically a Smurf who has been infected with the "purple disease" by either a purple fly or by another Purple Smurf, usually by biting him or her in the tail. A Purple Smurf usually hops around with gritted teeth, making "gnap gnap gnap" noises and searching for his or her next victim. A Purple Smurf can be cured by inhaling the pollen of a tuberose blossom, causing him or her to sneeze and restoring him or her back to normal.
The Purple Smurfs made their first appearance in "The Purple Smurfs". They are based on the original comic books' Black Smurfs, who first appeared in the comic book story "The Black Smurfs". It is probably due to the nature of the characters that they were made purple for the racially-cautious English-speaking audience, both for the cartoon show episode and for the English translation of the original story.
The comic book story "The Egg And The Smurfs" suggests that Grouchy's behavior has come from being the first Smurf turned into a Black Smurf and somehow some of its effects stuck with him when he was cured. This explanation does not appear in the cartoon show since "The Purple Smurfs" took place after "The Smurfette" in its history. In the story book version of "The Black Smurfs", Lazy Smurf is identified as the first Smurf infected with the disease.
A second appearance, although unofficial, was later in a one page Gag where Jokey Smurf painted himself black and went around the village in full Black smurf fashion causing panic with the others who believed that a second epidemic was about to break out. The joke however was on him as Handy Smurf started asking around who had taken his black paint, remarking that the paint will never come off. In the version that appears in the "Romeo And Smurfette" comic book, the black paint was changed to green, and in the Papercutz version that appears in "The Return Of The Smurfette" it was changed to purple.