More than 2,000 women have been warned that they may have contracted hepatitis C after being treated by a gynaecologist suffering from the virus.
Patients are being urged to take blood tests
Patients from 25 hospitals across England and two Scottish health boards were told of the scare in letters from the Health Protection Agency (HPA).
They have been asked to take a blood test to make sure they are safe.
The infected gynaecologist has been transferred to an area of healthcare where patients are not at risk.
The doctor, who has not been named, had not originally known he or she had the virus, which can lead to chronic liver disease.
Although hepatitis C does not often reveal symptoms, the virus can cause abdominal pain and jaundice.
In rare cases it can cause cancer of the liver.
Patients at risk have been asked to call a confidential helpline for further information.
Dr Fortune Ncube, from the HPA, said the risk of infection was "very small" and that screening was being offered as a "purely precautionary measure".
"All parts of the NHS involved have carefully identified and contacted any patients where the health care worker was involved in their care.
"Although the chances of having been infected are very small, I would advise patients to take a test if they have received a notification letter."
People could have hepatitis C without knowing it and modern treatment could prevent the onset of severe liver disease for many patients, he added.