49-year-old actress Christina Applegate revealed on Twitter this week that she has been diagnosed with multiple sclerosis (MS), an autoimmune disease affecting the central nervous system. Applegate says she was diagnosed “a few months ago” after experiencing symptoms of the disease.

Commenting on her diagnosis, she said: “It’s been a strange journey. But I have been so supported by people that I know who also have this condition. It’s been a tough road…but as we all know, the road keeps going.”

According to John Hopkins Medicine, multiple sclerosis occurs when the immune system attacks nerve fibers and the myelin sheath – a fatty substance which insulates healthy nerve fibers – in the brain and spinal cord. This attack causes inflammation, which destroys nerve cell processes and myelin, altering electrical messages in the brain.

There are different types of MS, the most common of which is relapsing-remitting multiple sclerosis, which affects 90% of those diagnosed. Symptoms of a multiple sclerosis relapse include: fatigue, numbness, tingling, blurred vision, unsteady gait, and weakness.

Worldwide, more than 2.3 million people live with MS, including almost 1 million adults in the United States alone, according to the National MS Society. The neurological autoimmune disease can be disabling, although the MS Society states that the majority of people with the condition do not become severely disabled. Two-thirds of people who have MS remain able to walk, though they may need a mobility aid, such as a cane, and some will use a scooter or wheelchair because of fatigue, weakness, balance problems, or to assist with conserving energy. 

Since coming out as newly diagnosed with MS, Applegate has received an outpouring of support from fans and other celebrities with the disease. Fellow actress Selma Blair, who co-starred with Applegate in a romantic comedy in 2002 and also has multiple sclerosis, tweeted: “Loving you always. Always here. As are our kids. Beating us up with love.” Talk show host Montel Williams, who also has MS, also tweeted his support: “We have MS – it will never have us unless we let it. Tara and I are sending hope and light your way.”

MS isn’t the first health battle Applegate has faced. In 2008, the star revealed that she had had a double mastectomy after testing posting for the BRCA gene, pre-disposing her to breast cancer. Facing her new MS diagnosis, Applegate has requested “privacy…as I go through this.”