One of London's top hospitals has written to 542 patients who may have been exposed to Hepatitis B.
Patients will be offered blood tests
The Royal London Hospital in Whitechapel, east London, has offered tests for the virus.
The alert was triggered after a patient, who has made a full recovery, was admitted last November suffering from the contagious liver disease.
A hospital spokeswoman said the risk of the patients contracting the disease is "extremely small".
She added that they have been contacted only as a precaution.
Royal London's spokesman said: "Hepatitis B is a notifiable disease, and whenever anyone is diagnosed with it public health authorities are obliged to assess the risk to other people, and to contact them for testing if necessary.
"Any patient testing positive for Hepatitis B will be given specialist advice and treatment."