Richard Marx Details His 'Terrifying' Battle with a Weeks-Long Mystery Illness: 'I Thought I Was Going to Die'

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."

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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.'"

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When Huizenga couldn't figure out what was plaguing Marx last year, "the only remaining possible answers were really grim," says the singer, who notes in his new memoir, Stories to Tell, out July 8, that lymphoma and a heart infection were listed to him as possible causes.

"The fear of getting that phone call or getting that diagnosis was real," he says. "Then, I just stopped being sick."

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When his symptoms eased about two months after they arrived last November, Marx says he had a new appreciation for life and for his wife of 6 years, Daisy Fuentes, who was a major source of support through it all.

"Whenever we'd talk about the [possible diagnoses] she would go, 'We don't know what it is, but I'm telling you right now it's not that,'" he says. "She just really believed that. She owned those thoughts. It wasn't just wishful thinking, she had a certainty about it. So now, of course, she likes to gloat - 'I told you you'd get better!'"

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Now Marx says he's feeling "100% great and healthy" and looking forward to all that's still to come, including the release of his new memoir and getting back out on the road with Fuentes, 54, by his side.

"It's definitely made a mark on me and us as a couple," he says of the illness. "So I'm going to really keep trying to enjoy my life more than I ever have."

For all the details on Richard Marx's life and love story with Daisy Fuentes, pick up the latest issue of PEOPLE, on newsstands everywhere Friday.