13 aka Remy Hadley has Huntington's Disease. If symptoms begin to show, like they did, in the early 20s it is likely she'll die by the age of 35. Until then, there will be a gradual loss of function and demetia.
According to an article in imdb, Olivia Wilde is only set to appear in the first few episodes and will take an indefinitive leave.
If this means that Cameron comes back, then I'm all for it!