Didier Drogba has been suffering from malaria for at least a month.
The 32-year-old Chelsea striker first complained of feeling unwell before the October international break but the illness was only diagnosed on Monday.
He did not start at Liverpool on Sunday after suffering a fever but played the second half of the 2-0 defeat.
Blues manager Carlo Ancelotti said he was unsure how Drogba picked up the tropical disease, but is now fine and will start against Fulham on Wednesday.
"We found out [on Monday] when tests showed he had malaria," Ancelotti stated.
"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 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.
"It was serious once. Now it's a bit different and you can treat it well and move on very quickly."
Chelsea said they cannot pin down where and when Ivory Coast international Drogba became infected.
He was already feeling unwell before flying to Africa to watch his country take on Burundi on 9 October.
A fever then ruled him out of the 0-0 Premier League draw at Aston Villa on 16 October and the following week's Champions League trip to Spartak Moscow.
Drogba has played four times for the Blues since then but has not scored from open play.
Having finally diagnosed the problem, Chelsea have insisted the striker will be firing on all cylinders within days.
"The course of medication he is on now will clear it up in three days," a club spokesman commented. "This type of malaria will not come back.
"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."
Drogba will lead the attack when Fulham visit Stamford Bridge with Michael Essien also returning in midfield.
"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."