Patient records over the last five years for people seen at Moberly Medical Clinics, Moberly Physicians Corporation and Moberly Rural Health Clinics were taken in a criminal cyber attack by a foreign-based intruder.
The parent company, known as community health systems, says its network was attacked in April and June of this year.
The data breach affected about 4.5 million people across the county.
Some of the stolen information included patients' names, social security numbers, phone numbers and addresses. The company says the theft did not include credit card information or medical records.
In a statement, Moberly Regional Medical Center said, "We take very seriously the security and confidentiality of private patient information and we sincerely regret any concern or inconvenience this even may cause for our patients."
Mandiant, a security company hired by CHS, believes the attacker was a group based in China that was most likely after medical device and equipment data.
The company immediately filed a report with the U.S. Securities Exchange Commission and removed the malware from its systems. They have also taken precautions against future breaches.
CHS has more than 200 locations nationwide with four of those being in Missouri.
All of the patients affected by the data breach will be notified by August 30 through mail. CHS will offer them free identity theft protection. There is a toll-free number 1-855-205-6951 set up for patients with any concerns or questions.