Chelsea have been stunned to discover that Didier Drogba has been suffering from malaria for at least a month.
Drogba complained of feeling unwell prior to the last international break in October and missed the league game against Aston Villa and the Champions League trip to Spartak Moscow.
The Ivory Coast striker also did not start Sunday's game at Liverpool, although he came on as a substitute at the start of the second half.
He was finally diagnosed as suffering from malaria last night when test results revealed a significant level of parasitic activity in his blood.
Drogba, who will play against Fulham tomorrow night, has been treated with a course of medication that should rid his system of the disease within days.
"He had a problem," coach Carlo Ancelotti said. "He has this virus and, obviously, he lost power and training.
"He lost his condition. He's had treatment and now he's okay. 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 now he has malaria. It's over now.
"It was serious once. Now it's a bit different and you can treat it very well and move on very quickly.
"Didier was, for this month, having difficulty to train. He didn't feel good. He was unselfish by playing when he was not at 100%. Now he has the possibility to improve his condition without a problem. He will play tomorrow."
A spokesman for the club added: "The course of medication he is on now will clear it up in three days. This type of malaria will not come back. We are not sure where and when he picked it up.
"It can lie dormant in the body for a long time. He is safe to play with malaria.
"When he first fell ill, we treated him symptomatically. He had a virus and it has come up and gone down, and then come up again.
"We ran tests on all the tropical diseases and viruses but because it was dormant, there was not enough parasitic activity in his blood to pick up malaria in the initial tests.
"You re-test and re-test and when the results came back last night the parasitic activity in his blood had reached a level where it showed up."
Although Drogba did not play for the Ivory Coast during the last international break, he went home to watch them.
But Chelsea say they are unlikely to ever pin down when and where he became infected.
Drogba will lead the Chelsea attack tomorrow night, although the Blues have fresh injury worries over striker Nicolas Anelka and defender Alex.
"We have to take a decision on Alex and Anelka," Ancelotti added. "Anelka has a little problem on his back and Alex has a little swelling on his knee.
"We'll take a decision tomorrow. The other players are in good condition. Michael Essien is okay."
Chelsea's 2-0 defeat at Liverpool has cut their lead at the top of the table to just two points, but Ancelotti is not pressing the panic button just yet.
"We didn't play well in the first half," conceded the Italian. "We did in the second. Now we have to look forward.
"We can improve our position in the next two games and we hope to move on from the defeat against Liverpool.
"We have to be able to play 90 minutes with the same intensity, with the same power and personality. That's our aim in the next games.
"We are still at the top of the table and we know this gap can improve or go down, but it's important to stay at the top of the table.
"Manchester United have a derby. We have one against Fulham and we have to pay attention but, in theory, we have a possibility to win.
"I'm not thinking about losing. I'm thinking about winning. It's important we do to move on from the defeat."