Lyon had been diagnosed with Stage 3 breast cancer in 2005 and died at age 37 on Tuesday, CBS has confirmed.
"Jenn died at home in Oregon at 11:19 p.m. on Jan. 19. She had been battling breast cancer for the last five plus years," former Survivor: Vanautu and Survivor: Micronesia -- Fans vs. Favorites castaway Eliza Orlinstold TVGuide.com. "She had beaten it into remission once, but then it came back and was in her bones."
"Jenn Lyon brought more good to this world in her almost 38 years than most will in a lifetime," Orlins wrote in a late-night Twitter message only a few hours after Lyon's death. "Jenn, I'll miss you. R.I.P."
Lyon -- who was billed as a nanny and graduate student from Encino, CA when she competed on Survivor: Palau -- had finished fourth on the Survivor tenth edition but was perhaps best remembered for her island romance with fellow castaway Gregg Carey.
According to Lyon, who had gotten saline implants six years earlier, she had initially detected some breast lumps in Summer 2004 (before Survivor: Palau was filmed in late 2004), but had failed to immediately follow-up with a physician.
"At the time I noticed the lumps I didn't have insurance, which was a big part of why I didn't get it checked immediately. I went on the Internet and thought, 'It is scar tissue [related to my breast implants]. No big deal,'" she wrote at the time.
"But a year later, I felt another lump right in the center of the breast and something in my right armpit. I saw a surgeon who said, 'I'm pretty sure you have breast cancer.' After a mammogram, an ultrasound and a biopsy, it was confirmed -- stage III."
Lyon subsequently underwent a modified, radical bilateral mastectomy in which both her breasts and 29 lymph nodes were removed, followed by chemotherapy.
"Jen was a genuinely selfless, sweet-natured person," Carty toldUs Weekly on Wednesday. "She always had a smile. She cared about every person."
"She was engaged in the greatest fight of her life, but she had every intention to fight and spread her positive messages," he added. "This has really rocked the Survivor community. It's just so sad."
Lyon had remained active in breast cancer fundraising efforts throughout her own personal battle, according to Orlins.
"The whole time she maintained a positive attitude. She was truly an amazing girl and I was lucky to know her," Orlins told TVGuide.com. "Even just last week when I saw her at the 10-year reunion, we were all laughing and she had her same glowing smile on her face."