An article by David Talbot:

"Software-controlled medical equipment has become increasingly interconnected in recent years, and many systems run on variants of Windows, a common target for hackers elsewhere. The devices are usually connected to an internal network that is itself connected to the Internet, and they are also vulnerable to infections from laptops or other device brought into hospitals. The problem is exacerbated by the fact that manufacturers often will not allow their equipment to be modified, even to add security features.

"In a typical example, at Beth Israel Deaconess Medical Center in Boston, 664 pieces of medical equipment are running on older Windows operating systems that manufacturers will not modify or allow the hospital to change – even to add antivirus software – because of disagreements over whether modifications could run afoul of U.S. Food and Drug Administration regulatory reviews..."

SOURCE

http://www.technologyreview.com/news/429616/computer-viruses-are-rampant-on-medical-devices/