Fiorina made her first public appearance at McClellan Business Park in suburban Sacramento. She thanked supporters for their well wishes and dismissed any concerns that her health would be a factor in her close race against Democratic Sen. Barbara Boxer.
"It's fairly common, unfortunately, for women who have been through the kind of breast cancer and reconstructive surgery that I went through to sometimes get infections," said Fiorina. "They need to be treated aggressively and quickly, and this one was, and I feel fantastic."
Fiorina had been forced to cancel campaign appearances in Riverside and Coachella on Tuesday. She missed a scheduled appearance in San Diego on Wednesday.
In Orange County, Fiorina spoke to supporters Thursday.
"The government works for us, not the other way around," said Fiorina in Laguna Woods Thursday. "And when someone has been in Washington, D.C., a really long time and forgotten who they work for, it's time to send them home."
Boxer was in Sacramento Thursday visiting a manufacturing company. She stuck to her message of clean-energy jobs.
"If we just keep on track in making America a leader and making California the hub of the new clean-energy economy, your future is as bright as the sun," Boxer said. "And that's what we're about."
The results from an exclusive Eyewitness News Survey USA poll, taken before the news of Fiorina's hospitalization, showed incumbent Boxer has 45 percent of the votes, while Fiorina has 40 percent.
Fifteen percent of those polled said they were still undecided or plan to vote for another candidate.
The Associated Press contributed to this report.