Oprah Winfrey has revealed she battled a "very serious" health scare that left her in the emergency room and forced her to cancel work appearances.

The 65-year-old appeared on The Ellen DeGeneres Show on Monday where she told the audience she was recently diagnosed with pneumonia.

She explained: "I came back from overseas and I thought I had a cold, but it wasn't a cold. I ended up in the emergency room and they said 'you have pneumonia'. 

"Pneumonia is nothing to play with y'all. It was very serious. I was on antibiotics for a week but the antibiotics weren't working.

"Then I went back for another CT scan and they said 'it's actually worse, you should see a lung specialist'. So I go into the lung specialist, I said 'I've got a little rattling'.

"He puts his stethoscope here [over her heart] and I see the 'Oh s***' face. 

"It's like, 'Oh my, something's wrong with you,' and I can see it, and he didn't hide it. I said, 'I told you it sounds like a rattlesnake in there.'"

The media mogul said her doctor urged her to do something she wasn't familiar with doing; canceling engagements in order to rest and recover.

She continued: "He immediately said, 'You must cancel everything.' I've never canceled anything in my life. I work when I'm sick. 

"He said, 'You can't fly anywhere for a month' and then he had 18 vials of blood drawn and I thought, 'this must be serious because I saw his face.'"

Winfrey said that she recovered after a week of being on antibiotics, and revealed her doctor was so happy about her full recovery that he asked for a hug.

She said: "He called me every day to make sure I was using the inhaler and taking the right antibiotics, and when I walked in and I was better, he thanked me three times for getting better, and said, 'Can I have a hug?' 

"And I could tell he was like, 'Not on my watch is this going to happen.'"

The TV star added that the ordeal has "changed" the way she looks at her health and wellness.

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