MS is a lifelong condition that affects the brain and nerves, and causes symptoms including fatigue, vision problems and issues with balance and coordination.
In an interview with Vanity Fair, Applegate, 51, said the condition means she “can’t even imagine going to set right now”.
“This is a progressive disease. I don’t know if I’m going to get worse,” she said.
Applegatebecame a household name when she was a teenager when she took on the role of Kelly Bundy in Married… With Children.
She is also known for playing Veronica Corningstone in Anchorman, as well as her more recent role as the widow Jen Harding in Dead To Me.
Image: Will Ferrell, Paul Rudd, Steve Carell, Christina Applegate and David Koechner at the UK premiere of Anchorman 2 in 2013
The final season aired on Netflix last November after Applegate was diagnosed while filming the final series.
Speaking about that time, she said she was “relieved that I no longer have to push so hard to get through my day” and described acting opposite Linda Cardellini.
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“Linda and I, from day one, were in love with each other and trusted each other and supported each other.
“I’m probably not going to work on camera again, but I’m so glad that I went out with someone who is by far the greatest actress I’ve ever worked with in my entire life, if not the greatest human I’ve ever known.”