Quite obvious what Carol's 'illness' was.
Besides being trapped in 1980's Southern California of shopping malls, gated communities and laundromats, Carol's ailment was being part of an unloving society. You could say that her family and friends at least passively cared for her well being, but they also simply saw her a piece of furniture and not as a human being with real issues or feelings. She was continuously let down and duped by others, eventually falling victim to a new age health guru scam that sought to feed off of her malaise (and perhaps her bank account) by selling her empty 'self help' nonsense. How can you learn to appreciate yourself for who you are when no one else really sees you? It's enough to send anyone into a self induced convulsion.
This is a dark warning tale of a world without love, much like Todd Hayne's other film, "Far from Heaven".