When Janet Jackson was rushed to a hospital in Montreal shortly after arriving for one of her shows during her comeback tour last fall, many speculated she was suffering from exhaustion or, worse, low ticket sales.
But when she cancelled six concerts and her doctor advised her not to go on stage, it was clear that Jackson's mystery illness was serious. According to a statement released by her publicist, Jackson suffers from a "rare form of migraine headaches called vestibular migraine or migraine-associated vertigo."
Doctors say Jackson's type of migraine is well documented, but it only affects 3 to 5 percent of the general population. Why it happens remains somewhat of a mystery.
"Twenty to 30 years ago, this used to be called floating women's syndrome; [doctors] used to consider it a psychiatric or neurotic syndrome," Dr. Steven D. Rauch, a professor of otology and laryngology at Harvard Medical School and a doctor at Massachusetts Eye and Ear in Boston, told ABCNews.com.
Rauch said the symptoms of vestibular migraines may mimic the spinning in classic vertigo, or patients might just constantly feel off balance.
"Patients feel like they have no balance, or they feel like they're rocking on a boat all the time, like you're lost in space," said Rauch.
Comedian Roseanne Barr has claimed to have had many illnesses, but probably her oddest one would have to be multiple personality disorder.
In 2001, she told Larry King that she has suffered from the disorder since childhood and, even after 12 years of psychotherapy, sometimes regresses.
"I often acted irrationally and I still often do act irrational," she said. "Like, OK, I had glasses. And sometimes I needed glasses and sometimes I didn't. And I never thought about that. I'd just take them on and off. And I would be allergic to different things, like codeine and then sometimes not allergic. And you know, different things like that."
Barr said she would switch personalities so often and so subtly that she would sometimes forget how to drive while she was driving.
"I'd have to pull over off the side of the road and call people to come and get me," she said.
When King asked Barr how she avoided the "nuthouse" with her disorder, she shot back, "I'd never avoid the nuthouse. I've been hospitalized several times. You know that, Larry."
Some people are clumsy and others have dyspraxia.
"Harry Potter" star Daniel Radcliffe says he suffers from a mild form of the latter. Dyspraxia is a brain disorder that is often associated with clumsiness because it can impair coordination. It can affect language and intellectual development as well as physical. People with a severe case may find it difficult to walk up or down the stairs, kick a ball or run.
For Radcliffe, it means he still has trouble tying his shoelaces.
"I sometimes think, 'Why, oh why, has Velcro not taken off?''' he told Britain's Daily Mail last year.
"Yes, Dan Radcliffe does have dyspraxia. This is something he has never hidden. Thankfully, his condition is very mild and, at worst, manifests itself in an inability to tie his shoe laces and bad handwriting," his rep told an Australian newspaper.
Radcliffe revealed to the Daily Mail that he became an actor in part because his dyspraxia meant he struggled in school. "I was having a hard time at school in terms of being crap at everything with no discernible talent."