A hacker has pleaded guilty to infecting thousands of computers with a botnet programme, including those belonging to a hospital network and the US military.

The malware released by 20-year old Christopher Maxwell in January 2005, is said to have caused a number of problems for computers in Northwest Hospital and Medical Center in Seattle, the most serious of which was to close down computers used by the intensive care department.

Other affected systems included those for controlling operating room doors, and the paging system used by medical staff. The chaos would have been more severe were it not for backup systems.

The malware also affected the Department of Defense computer systems in Germany and 400 PCs belonging to the Directorate of Information Management in Colorado.

The accused could now face a prison sentence of up to 15 years, though he is reported to have agreed a $252,000 compensation arrangement with the hospital as part of his plea.

The intention of the botnet was to install unauthorised adware, a job for which the accused and two juvenile co-conspirators are said to have been paid $100,000 in advertising commission.