Christina Aguilera, singing very sick

There's something that impresses me in professional singers with big voices: their endurances. Here is a video of Christina Aguilera singing with Chris Mann in the final of The Voice season 2, where she was her vocal coach. 
She was sick like a dying dog, she couldn't even speak that night. However when it was her turn to sing with her pupil on the show she was awesome as always, no one would have ever imagined she was almost voiceless that night.
To see this unforgettable performance click the Here.
Enjoy!