What could have happened to her?

My younger sister (who's 16) has a teacher at school who has been off "sick" since late March, early April. She was only supposed to be off for 2-3 weeks before she came back, but nobody's seen her since. They've all asked their new teacher, other staff what's happened to her (just out of pure concern) and they "don't have the liberty to say"... She is apparantly off sick and isn't expected back at school until at least October (but probably won't be back until next year).

Does anyone know what sort of medical problem could be expected to last a few weeks except blow into a 10 month ordeal. Some thoughts we had were botched surgery, cancer of some sort, back issues etc. The puzzling part is the "not at liberty to say" because if it was just back issues or something they wouldn't mind saying... She's in her 40's, female, no kids, got married a couple of years ago...

Any other ideas as to what might have happened. My sister and her friends are just worried about it, and she used to teach me too, so I have a certain amount of concern. Thanks for all your help!!