As many as 40,000 people who used a Las Vegas clinic are being urged to be tested for HIV and the blood-borne hepatitis C virus, US officials say.
Anyone who received anaesthesia injections from the Endoscopy Center of Southern Nevada from March 2004 to January 2008 should be tested.
The warning came after an investigation found the centre had been responsible for "unsafe injection practices".
Hepatitis C can cause fatal liver problems and is very hard to treat.
People with the infection can pass it on if their blood gets under the skin or into the bloodstream of another person, for example through the use of a shared syringe.
The Southern Nevada Health District announced on Wednesday that it had identified six cases of hepatitis C at the clinic, five of which stemmed from procedures on the same day in September involving anaesthesia.
The sixth patient is believed to have been infected in July, the health district said.
Following an investigation, the district determined that "unsafe injection practices related to the administration of anaesthesia medication might have exposed patients to the blood of other patients".
It said a syringe that was used to administer anaesthetics to one patient may have contaminated the vial from which the anaesthetics were drawn.
The vial, which was not intended for use on multiple patients, was subsequently reused. If the vial was contaminated with hepatitis C, it could have exposed others to the blood-borne pathogens such as hepatitis C, it added.
The district said it was advising patients to contact their doctors and get tested for hepatitis C, hepatitis B and HIV.
The Endoscopy Center said in a statement that it had now changed its practices.
"All concerns noted by the health department were addressed immediately. We want to be sure that every patient who may have been exposed is informed and tested," it said.