WHEN BRATTY NELLIE OLESON CAME TO Walnut Grove, Minn., in August 1992 for a Welcome Home Nellie picnic, it was on one condition. "I said, 'I'll stick my feet in Plum Creek,' " recalls Alison Arngrim, 31, who played Nellie for seven seasons on Little House on the Prairie (set in Walnut Grove), " 'but I have to be able to speak about AIDS.' " She did so—and also raised money that day for a Minneapolis AIDS hospice.

AIDS education has become Arngrim's life mission ever since her close friend and Little House costar, Steve Tracy—who played her husband, Percival Dalton—died of the disease in 1986 at 34. Alison, who has seldom acted since leaving Little House, volunteers with AIDS Project Los Angeles and Tuesday's Child, a pediatric AIDS organization, and assists four HIV-positive "godchildren." She also tours the country giving safe-sex talks in schools, churches and prisons. "Some of my biggest fans are in prison," says Arngrim.

Arngrim was born into show business; both her manager father Thor and mother Norma MacMillan are actors. The day after Little House debuted in 1974, one of her classmates at Bancroft Junior High in Hollywood yelled playfully, "You bitch!" "I took a bow," says Alison, then 12. "I was doomed."

In her new guise, maybe destined. The soon-to-be-divorced Arngrim is even dating a comrade from the AIDS front, Robert Schoonover, 44, the former head of the Southern California AIDS Hotline. And while she's eager to return to acting, the rewards of her current job are as clear to her as Plum Creek. "Nellie got to come home and help fight AIDS," she says. "Percival would be proud."