The USA soccer star played through the illness in the match versus Mexico in Azteca.
He had apparently been feeling ill since the USA squad met for training camp in Florida.
"I felt something when I got into Miami on Sunday," Donovan told SI. "I just felt real bad and finally went to see the doctor. They took a swab from my nose and confirmed it, so that was that."
On Wednesday, Donovan played for 90 minutes in Azteca stadium in the World Cup qualifying match versus Mexico. Though he assisted on the Charlie Davies goal that put the USA in the lead in Azteca for their first time ever, Donovan also tired and trailed defensively on the play that led to Mexico's winning goal in the late in the match.
"I've never felt that bad," said Donovan. "The last time I was there [at Estadio Azteca] I didn't feel that bad. I just felt lethargic, slow. I didn't feel normal."
Though Donovan thought he might feel well enough to play in the Galaxy's weekend match to the Seattle Sounders, a physician told Goal.com that the MLS squad would have to be careful to keep the player from spreading the virus to his teammates, who include England star David Beckham.
According to the Center for Disease Control, persons with H1N1 may no longer be contagious a week after illness onset, providing that their symptoms (fever, chills) have resolved.
The Galaxy are making a run for the MLS playoffs, but the Seattle Sounders are fellow Western Conference rivals and will look to gain points if Donovan cannot play.
The Galaxy player has been in good form this season, leading the team in goals scored.