Keira Knightley battled pleurisy in 2012

Keira Knightley was struck down with pleurisy last year after her hectic work schedule made her ill.

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 tells Australia's Harper's Bazaar magazine, "You do period films and you get some sort of weird archaic disease. And I didn't realize, 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?'"