Social Security numbers of some Grand Blanc, Mich.-based McLaren Health Care patients were affected by an August breach on its computer systems.
In a breach notification posted to its website, the health system said that around Aug. 22, it became aware of suspicious activity linked to specific McLaren computer systems.
McLaren launched an investigation into the incident and learned that between July 28 and Aug. 23, an unauthorized person had obtained certain information stored on the network.
The investigation, which concluded Oct. 10, revealed that the unauthorized person accessed patient information such as Social Security numbers, health insurance information, dates of birth and medical information, according to the notification.
McLaren said it is sending notice letters to affected individuals. According to the HHS data breach notification site, 501 individuals were impacted.