While Michael Landon had nine children of his own, he was a father figure to many others, including his Little House on the Prairie costar Melissa Gilbert. So, it was just as hard for her when Landon was diagnosed with pancreatic cancer and died three months after announcing his diagnosis to the world.

This is the thing that killed two of the strongest, most extraordinarily extraordinary physical specimens of men I'd ever known in my life,” the actress tells PEOPLE

Michael Landon, who starred in "Bonanza" and "Little House on the Prairie," died in 1991 at age 54. The cause of death was pancreatic cancer. He died three months after announcing his diagnosis.