Christina Applegate shares a message on her 50th birthday after MS diagnosis - CNN

It's considered an autoimmune disease in which the immune system attacks its own healthy cells.

"But I have been so supported by people that I know who also have this condition," Applegate said.

But as we all know, the road keeps going."

There is no cure for multiple sclerosis -- known as MS -- but many people are able to manage their symptoms and adapt to new lifestyles after diagnosis.

Researchers believe that the disease is the result of both environmental and genetic factors, but the underlying cause is not known.