“Around 2.9 million people worldwide—roughly one million in the United States—have multiple sclerosis or MS; Applegate and The Sopranos star Jamie-Lynn Sigler are two of them. In their podcast, the actresses speak candidly about their experiences with the disease.
MS affects the central nervous system, the brain and spinal cord. The immune system attacks sheaths of a material called myelin that surround nerve fibers. Like insulation on wires, myelin protects nerves and helps transmit signals. But when myelin deteriorates, the nerve fibers underneath are exposed. This disrupts the brain’s communication and leaves behind lesions, which can cause a myriad of symptoms as the disease progresses.”

https://www.nationalgeographic.com/science/article/multiple-sclerosis-ms-cause-treatment-ebv