Rosenbaum, who was featured on the programme’s seventh season, explained in a series of Instagram Stories videos filmed from the hospital that he has Guillain-Barré syndrome.
He called the diagnosis “surreal and humbling”.
“Things you do every day like picking up this phone or buttoning buttons, tying shoe laces, putting on deodorant… Just can’t do it,” he said.
Guillain-Barré syndrome affects the nerves and can cause symptoms such as muscle weakness, pain and numbness.
Most people fully recover, but some are left with long-term symptoms.
Rosenbaum explained that he and his treatment team would work on establishing whether his symptoms have plateaued and that he will go through physical therapy.
“Hopefully I’m on the road to recovery,” he said.
Rosenbaum, 42, married The Bachelorette‘s Ashley Hebert in 2012 after they both appeared on the reality programme.
The couple have two children together.