So anyone know what's wrong with Kate?

Best theories I've heard so far are...

Crohn's disease (explains abdominal surgery and long recovery time)

Mental Health issue (explains why she's only seen in fuzzy long shot / hidden in an attic like Mr Rochester's wife)

Lazy Cow (pretends to be ill but still pottering about as normal while on a sicky)

Estranged (has the hump with William so gone on strike)

**

Update: The correct answer was that like 1 out of 2 people during their lifetime, Kate sadly has cancer.  It comes to every other one of us....