The Crown star Emma Corrin secretly battled whooping cough while filming the royal drama series.
Corrin told Interview magazine she was only able to recover from her bout of illness after a trip abroad to film her final scene was cancelled earlier this year.
Whooping cough is a bacterial infection of the lungs and breathing tubes, with symptoms including bouts of coughing and gasping for breath. While it can be most dangerous in babies and children, adults can still suffer nosebleeds and sore ribs and may be treated with antibiotics.
The revelation comes after Emma, who also suffers from asthma, shared that she was rushed to hospital while filming the Netflix show in Spain due to "low oxygen levels" after a swimming pool scene.
"I'm asthmatic and had been ill for a while with a cough," she previously told Glamour U.K.
"I had to film a scene in a freezing cold swimming pool. It was the hardest scene to film because I was genuinely keeping myself alive treading water," she revealed.
Corrin went on: "We were meant to be flying back that night to the U.K. and went past a hospital to get antibiotics. The doctors gave me an oxygen test and said, 'We can't let you go because your oxygen levels are so low', so I was hospitalised."
Season four of The Crown, which also stars Gillian Anderson as Margaret Thatcher and Olivia Colman as Queen Elizabeth II, is streaming now.