Arngrim's other best friend on the set was Steve Tracy, who played her on-screen husband, Percival Dalton, toward the end of the series. When Tracy contracted AIDS and went public with his diagnosis in the mid-1980s, Arngrim stood by him. She began volunteering for AIDS Project Los Angeles and speaking about the disease.
"Back then, people were even terrifed to have a presentation about the disease, but they'd say, 'Oh, it's the lady from "Little House."' I got into places and talked about HIV and AIDS where they would never have someone speak," Arngrim said.
Tracy died in 1986, but Arngrim continued to be an activist. Through AIDS Project, she met her husband Robert Paul Schoonover, who plays in the Southern California band Catahoula, and they've been married 17 years. She also got involved with the National Association to Protect Children.
While fighting to change a California law that allowed incest offenders to leave jail early, Arngrim decided to go public with her own childhood sexual abuse in a 2004 interview with Larry King.
"I remembered Steve going public," she said, "and thought the shoe is on the other foot now."
That opened the floodgates for Arngrim, who began touring her comedic one-woman show "Confessions of a Prairie Bitch," in which she talks about her years playing Nellie.
Find out what happened to some of Arngrim's fellow cast members since the series ended.
Melissa Gilbert/Laura Ingalls: