richard marx

Richard Marx

| Credit: Nick Spanos

Richard Marx is opening up about a mystery illness that plagued him twice over the course of 13 months.

In this week's issue of PEOPLE, on newsstands Friday, the '80s pop star, now 57, details how he was struck with the illness - which caused him debilitating seizures, fevers and chills for weeks on end - for the first time in October 2019 and then again last November.

During the first round in 2019, "I thought I was going to die," Marx tells PEOPLE. "The violence of the seizures alone was terrifying. I remember thinking, 'I don't know if I'm going to get out of this bed, ever.'"

After visiting an emergency room in Montclair, New Jersey, where he was playing a set of shows at the time of that first bout, Marx did multiple blood tests and brain and full-body MRIs but everything came back negative. Then when he returned home to Los Angeles, he was treated by an infectious disease expert who said he had a "fever of unknown origin."

After his second bout with the illness, his doctor, Rob Huizenga, gave him four COVID tests that all came back negative. He still doesn't have a diagnosis.

"My doctor is one of my best friends, and we play tennis every Sunday," Marx says. "Just a couple weeks ago when we were playing, he said, 'I've got to tell you man, I've had a long career and you are the single biggest question mark in my career. I have no idea what the hell you had, but I'm here playing a hard game of tennis with you and I'm looking at you and I'm thinking, A year ago I was really scared you weren't going to stay with us.'"