Busch handed his car over to Denny Hamlin on lap 40 of the Nationwide race, which he started despite feeling unwell. He completed the first portion of the race as he had planned, in order to be credited with the result in the end. NASCAR rules allow a driver to be replaced and still get the points if he was behind the wheel at the start.
The 24-year-old was in the lead when he jumped out of his #18 Toyota, but said that the heat was starting to affect him while driving.
"It was just temperature," said Busch. "When I get in the car, and I get hot, I start not being able to see exactly straight."
Busch said he preferred to avoid running the whole Nationwide event in order to try to be in better shape for the Sprint Cup race.
"That was another problem, is wanting to get out today in order to be able to compete the full 500 miles tomorrow," Busch added. "Fortunately, we got Denny Hamlin who's our relief driver that can drive for me, and I can try to go back now, rest up, cool down, try to get this fever under control and be ready for tomorrow."
Team officials were hopeful that Busch will be fit enough to drive the full distance of the Sprint Cup race. However David Gilliland, who is set to drive for Joe Gibbs Racing in three races this year, is expected to be on standby for him.
After being handed Busch's Nationwide Series car, Hamlin went on to reclaim the lead before crashing in the closing stages after making contact with Greg Biffle.
A 31st place finish meant Busch lost 90 points of his 254-point lead to Carl Edwards in the Nationwide standings.
Valencia enter Jovanovic raceHamlin: Pocono win a major boost
