Menino went to his doctor Friday and was then admitted to Brigham and Women's Hospital, The Boston Globe reported.
"They are diagnosing it as a virus at this point," spokeswoman Dot Joyce said Sunday. "He's fine. He's tired. He is participating in decisions regarding the storm [Hurricane Sandy]."
Doctors ruled out pneumonia and bronchitis, Joyce said.
It was unclear when Menino will be released from the hospital, she said.
"We're hoping Monday or Tuesday, but we'll see," Joyce said.
Menino, who has been the city's mayor for 20 years, has been hospitalized about 12 times since he took office for various reasons, including two knee surgeries and Crohn's disease, a chronic inflammation of the intestines.