Authorities urged 4,500 people who were treated by an anesthesiologist to get tested for hepatitis C, saying three patients may have been infected as the doctor gave them anesthesia.

The city Department of Health and Mental Hygiene said Thursday it was mailing letters to everyone at risk and noted that the liver-damaging disease cannot be spread by casual contact.

Three people treated by the doctor in August were diagnosed with hepatitis C in recent months, the health department said. Laboratory tests suggest they were infected while getting intravenous anesthesia drugs during outpatient procedures, according to the agency.

Authorities have not identified the anesthesiologist. A state Health Department spokeswoman, Claudia Hutton, said the agency had not established "that the doctor is guilty of doing anything wrong."

The doctor has had a medical license since 1977 and "does not have a history of spreading infection," Hutton said.

City authorities said they were contacting everyone treated by the anesthesiologist while he or she practiced at 10 different medical offices in New York City, from Dec. 1, 2003, to May 1, 2007. The doctor has stopped practicing during the investigation, the city health department said.

"Transmission of hepatitis in a medical setting is rare, but as a precaution we are reaching out to anyone who could have potentially been exposed," Dr. Marci Layton, the agency's assistant commissioner for communicable disease, said in a statement.

She stressed that intravenous medications are "very safe when standard infection-control procedures are followed," and that patients should not avoid important procedures because of worries about infections.

Hepatitis C is a chronic, blood-borne virus that that can cause scarring or other damage to the liver. It often does not cause noticeable symptoms, although some people experience flu-like symptoms, a yellowing of the skin and the whites of the eyes, dark urine and pale feces. It is treatable, but many people who have the disease do not even know they are infected.