The Radioactive singer spoke candidly to People magazine in November (16) about his struggles with ankylosing spondylitis (AS), a type of arthritis which causes long-term inflammation of joints in the spine.
Dan admitted the illness had plagued him during the group's rise to fame, and really left him in a "bad place", especially as the rockers worked on their 2015 album Smoke + Mirrors.
"Right at the beginning of Smoke + Mirrors was really when A.S. was rearing it's head in a big way," he recalls in a new interview with People. "It was the beginning of the disease in a lot of ways for me and learning to manage it. So physically I was in a quite a lot of pain, and mentally I was in a very bad place as well."
Dan has since gotten to grips with the condition, which now allows him to better manage his symptoms, and as a result, he has been enjoying "a very healthy year" after much "physical and mental work".
Fighting through the health struggle has made him a more positive person, and now he's able to recognise it has only made him stronger.
"I'm grateful for it," he says. "Looking back in this last year from a place of health, you're able to have greater perspective and I've had more perspective to see that a lot of the great things in my life are due to struggle.
"A lot of my greatest strengths are due to my greatest weaknesses or flaws or physical ailments. It brought me discipline, gratitude and compassion."
Dan channelled his emotions about his illness into the band's new single Believer, making it a really personal track to perform.
"The song is about how pain made me a believer," he shares. "It's made me a believer in myself, it's made me a believer in my art and work. I wouldn't have my art if it wasn't for pain. It takes somewhat of a healthy place to appreciate it because when you're in the midst of it you don't appreciate it. You're just upset."
Despite Dan's health turnaround, he insists there are still days when he struggles with severe pain, but he is urging others suffering from similar ailments to keep pushing through.
"With depression or A.S., it's not just a pit for the rest of your life or this downward spiral. It's the beginning of something that is going to cause you to have to grow to make changes," he concludes. "While it can seem bleak right when you're diagnosed, there are management plans so that's why it's so important for it not to remain a hidden disease and that awareness is raised."