Didier Drogba was diagnosed as suffering from malaria on Monday night, Chelsea coach Carlo Ancelotti has confirmed.
The Ivory Coast striker was thought to have been suffering from a virus in recent weeks but blood tests revealed it was the tropical disease.
Drogba was left out of the starting line-up for the 2-0 defeat at Liverpool but came on as a second-half substitute after complaining of a fever on the eve of the game.
"We found out last night when blood tests showed he had malaria," said Ancelotti. "I don't know where he got it from."
Despite suffering from the disease, Drogba will start against Fulham at Stamford Bridge on Tuesday night.
"He had a problem," added Ancelotti. "He has this virus and, obviously, he lost power and training.
"He lost his condition. He's had treatment and now he's ok. He'll come back immediately in the best condition.
"He suffered, obviously, because he wasn't 100% but now, after treatment, he'll be better. He had malaria. He had the tests in the last day, and they showed this kind of virus. Now he's good.
"I don't know where he got it from. I gave him a few days off and he went on holiday for a few days, and then he has malaria. But it's over now. He's ok."