The 22-year-old US Open champion kept the "Race to Dubai" money list battle alive by winning the Hong Kong Open on Sunday, but has been struggling with his health for some time.
McIlroy went to see a doctor on arrival in the United Arab Emirates for the Dubai World Championship. It starts on Thursday and he needs to win, and for Donald to finish outside the top nine, to be crowned number one for the season.
"I won't get the results back until later on this week," said the Northern Irishman. "I picked up a virus and my white blood cell count was very low, so the tests were to see if it's back up again."
It is hardly surprising. McIlroy has been virtually non-stop since late September and that has included three trips to the Far East and one to Bermuda.
"I'm not energy-wise where I want to be, but I took a day off yesterday and took it easy," he added.
"I'm going to play the pro-am today and practise and I'll probably take another day off tomorrow - maybe hit a few balls.
"I'll really try and conserve my energy and focus it all into the four tournament days.
"I'm not 100%, but I'm still able to go out and play 18 holes and try and give it my all.
"I've still got a slim chance of trying to win the 'Race to Dubai'. It's going to be a tough ask, but I'll give my best and it would be great to finish the year on a high."
Not that he is quite finished yet. McIlroy is also entered for next week's Thailand Championship, although his manager Conor Ridge says his participation there depends on his health.
Asked if he might have pulled out this week if it had not been such a big event - not just for the money list, but also the world rankings and Ryder Cup qualifying points - he said: "I'm not sure.
"I don't like pulling out of tournaments. I think you saw that at the PGA this year." That was where he damaged his arm hitting against a tree root on the third hole, yet he managed all four rounds.
Tee-off times in the first round of the Dubai World Championship at Jumeirah Golf Estates, Dubai, on Thursday (All times GMT. Dubai is four hours ahead):
0340 Christian Nilsson (Swe), Scott Jamieson
0350 Hennie Otto (Rsa), Peter Lawrie
0400 Paul Lawrie, James Morrison
0410 Felipe Aguilar (Chi), Rhys Davies
0420 Ernie Els (Rsa), Jaco van Zyl (Rsa)
0430 Ross Fisher, Thorbjorn Olesen (Den)
0440 Lorenzo Gagli (Ita), Gregory Bourdy (Fra)
0450 Shane Lowry, Edoardo Molinari (Ita)
0500 Thomas Levet (Fra), Robert Karlsson (Swe)
0510 Robert Rock, David Horsey
0525 Johan Edfors (Swe), Fredrik Andersson Hed (Swe)
0535 David Lynn, Retief Goosen (Rsa)
0545 Jamie Donaldson, Raphael Jacquelin (Fra)
0555 Paul Casey, Rafael Cabrera-Bello (Spa)
0605 Mark Foster, Matteo Manassero (Ita)
0615 YE Yang (Kor), George Coetzee (Rsa)
0625 Richie Ramsay, Gregory Havret (Fra)
0635 Francesco Molinari (Ita), Michael Hoey
0645 Joost Luiten (Ned), Ian Poulter
0655 Thomas Aiken (Rsa), Alvaro Quiros (Spa)
0710 Nicolas Colsaerts (Bel), Pablo Larrazabal (Spa)
0720 Peter Hanson (Swe), Gonzalo Fernandez-Castano (Spa)
0730 Louis Oosthuizen (Rsa), Graeme McDowell
0740 Alexander Noren (Swe), Miguel Angel Jimenez (Spa)
0750 Darren Clarke, Simon Dyson
0800 Thomas Bjorn (Den), Sergio Garcia (Spa)
0810 Anders Hansen (Den), Lee Westwood
0820 Charl Schwartzel (Rsa), Martin Kaymer (Ger)
0830 Rory McIlroy, Luke Donald