Officials are saying that a virus is to blame for compromising thousands of patient medical records at Alberta Health Services.

The virus impacted AHS' network and Netcare, Alberta's electronic health record, from May 15-29 before it was detected and removed, said officials.

The virus is said to be a new variant of a Trojan horse program called Coreflood, which is designed to steal data from an infected computer and send it to a server controlled by a hacker. Coreflood captures passwords and data the user of the computer accesses.

AHS has identified two groups who are potentially at risk - patients whose health information was accessed in Netcare through an infected computer, and employees who accessed personal banking and email accounts from work using an infected computer.

Officials are sending letters to the 11,582 patients whose information may have been exposed and have notified all affected employees.