According to MS Australia, multiple sclerosis is "a condition of the central nervous system, interfering with nerve impulses within the brain, spinal cord and optic nerves. It is characterised by sclerosis, a Greek word meaning scars."
"These scars occur within the central nervous system and depending on where they develop, manifest into various symptoms."
It's not the first health battle Applegate has faced. In 2008 she had breast cancer and underwent surgery to have a double mastectomy.
She wrote her battle with breast cancer into her hit Netflix show Dead to Me.
"It's such a personal choice," she previously told USA Today. "I don't know what anyone's going to take away from that moment or scene, except for 'Wow, I felt that way, too, and I'm going to be OK'. Because you will. It's OK to admit that you feel that way."
Other stars who have been open about living with MS include Selma Blair, Jamie Lynn Sigler, and Jack Osbourne.
Applegate and Blair both starred in the iconic movie The Sweetest Thing together in 2002.
Applegate rose to fame playing Kelly Bundy in Married With Children.
