Marianne suffers from depression/bipolar disorder
- After Willoughby breaks up with her, she exhibits symptoms of depression: lack of appetite, lack of sleep, general disinterest in everything, lack of self-care (it's mentioned that Elinor has to prep her for when they go outside) and a general tendency to stay unoccupied in her bed all day. Moreover, when she learns how much Elinor has suffered, she still finds it impossible to adjust her conduct on her sisters', even as she feels her moral superiority. In the book this is treated as complacency, but it could be that it is genuinely impossible for her to "stiffen up", as it is for people who suffer from depression. As her state lasts more than two weeks, this could count as a severe depressive episode.
- At the same time, when she is happy, she acts with (for the time) recklessness - by openly displaying her affection and intimacy with Willoughby before they are engaged - which is a sign of a manic episode in someone with bipolar disorder. Marianne is certainly a self-absorbed teen, but it is possible that she also suffers from a mood disorder, which would explain how extreme her mood gets.
Marianne's illness is diphtheria
- See here
for an informed "diagnosis." Diphtheria seems to fit the descritption of her illness to a T: an infection that develops quickly, has a sore throat, cough, and high fever among the main symptoms, and can initially be mistaken for a cold, yet can be life-threatening. Of course the disease can't be caused by heartbreak or by being cold and wet, but those factors could have exacerbated it.
- See here
