FYI, here are the stats as they appear on the NMSS website. Of course, we don't know exactly how they were derived or how old this is -- remember that the 1.5 million worldwide/400,000 U.S. stats are from 1975.

"The average person in the United States has about one chance in 750 of developing MS. But relatives of people with MS, such as children, siblings or non-identical twins, have a higher chance—ranging from one in 100 to one in 40. The identical twin of someone with MS, who shares all the same genes, has a one in three chance of developing the disease."