The Big Bang Theory stars struggling with mystery bug

The Big Bang Theory cast has been struck with a nasty illness.

Two days after Kaley Cuoco posted a photo on Instagram of herself and her castmates sporting surgical masks on set, stating, "everyone at work is sick", co-star Kunal Nayyar has revealed his pals are all very poorly.

"Literally everyone is sick," he tells Entertainment Tonight. "We were rehearsing and we were doing scenes, and we were all wearing surgeon masks, doctor masks, because everyone was sick.

"I think, Mayim (Bialik) had it first, and then Kaley... Also everyone's got kids now. Simon (Helberg)'s got kids... Everyone's getting sick all the time. I think Mayim gets it from her kids, or Simon gets it from his. And they come to work and we're all hugging - we're all very close.

"It's funny. We were literally doing scenes (and we'd) take off the mask... and put it back on."

Things got so bad, the producers had to call in medics.

"We had to bring the doctor to set, like, three times," the Anglo-Asian star tells ET.

But he credits his Indian roots with helping him stay healthy as his castmates suffer: "I never get sick. It's because I'm from India, so I've seen every kind of bacteria you can imagine."

And Kunal admits co-star Kaley tries to keep him and the cast and crew healthy, when she's not so sick, by leading weekly yoga sessions.

"We have yoga Thursdays," he reveals. "At lunch, she teaches us all yoga on Thursdays."