It's not visible to the naked eye, but Natalie Cole has just begun the fight of her life. The Grammy winning singer is battling hepatitis C, and tonight on ET she's sharing her story for the very first time with her good friend and ours, Paula Abdul.

"In 1980-1982, I was doing heroin," reveals Natalie, tracing the possible cause of contracting the disease. "The first day that 'This Will Be' came out on the radio I was on my way to get heroin."

While she overcame her drug problems long ago, she's paying the price today.

"This is a virus that was dormant in my body for 25 years," she explains. "I had no symptoms, was feeling great, working out like a crazy person -- and this just keeled me over, like a feather."

Natalie has lost a lot of weight due to her struggle, but remains light-hearted about it: "I now tell people I'm runway size. I always wanted to be for 20 years; I've been waiting."

The good news is that Natalie's form of the disease is treatable.

"What I have is treated with chemo ... every week," she says, adding that she experiences the traditional side effects from the treatment, such as nausea and her hair falling out.

In addition to loving friends and family helping to boost her incredible spirit, Natalie of course has her music. Still Unforgettable, in stores today, comes 17 years after Natalie's award-winning "Unforgettable" duet with her late father, Nat King Cole, and they're singing together again with the help of a little digital magic.

"We wanted to do something that was fresh and flirty and fun with dad this time," she says, "and not concentrate so much on this being a record of Nat King Cole as it is to his era."

Watch ET for more of Paula's emotional interview with Natalie!