"I just honestly want to do what's best for my health," Borland told . "From what I've researched and what I've experienced, I don't think it's worth the risk."
Borland - who's suffered two diagnosed concussions - said he notified the team of his decision Friday after consulting with friends, family, teammates and concussion researchers.
"I'm concerned that if you wait till you have symptoms, it's too late. ... There are a lot of unknowns." Borland said after stating that he isn't currently experiencing symptoms. "I can't claim that X will happen. I just want to live a long healthy life, and I don't want to have any neurological diseases or die younger than I would otherwise."
Borland, November's defensive rookie of the month in 2014, was expected to be a big part of the 49ers defense. He tallied 108 tackles on the season and started eight games in Patrick Willis's absence.
"While unexpected, we certainly respect Chris’ decision,” 49ers general manager Trent Baalke said, according to Matt Maiocco of CSN Bay Area. "From speaking with Chris, it was evident that he had put a great deal of thought into this decision, he was a consummate professional from day one and a very well respected member of our team and community."
"We will always consider him a 49er and wish him all the best.”
Borland cited many former players who have experienced nightmarish personal lives after football in his decision. Degenerative brain diseases, like chronic traumatic encephalopathy, has been strongly linked to the type of repetitive brain trauma that football players experience.
"I've thought about what I could accomplish in football, but for me personally, when you read about Mike Webster and Dave Duerson and Ray Easterling, you read all these stories and to be the type of player I want to be in football, I think I'd have to take on some risks that as a person I don't want to take on."