The Atonement star thought she had injured her ribs while rehearsing for a show and battled through the pain, only to find out weeks later she had caught the agonising lung infection.
She told Australia's Harper's Bazaar magazine: "You do period films and you get some sort of weird archaic disease. And I didn't realise, so I went on for another month. I just stupidly never went to a doctor.
"I was rehearsing for this play and and then I found in rehearsals it got so bad that I couldn't stand up any more. And they said, 'Please, can you go to the doctor?'"