Mars Attacks? Protecting Earth From Off-World Infections

This article starts with the acknowledgement that there are several virulent bacteria on this earth, such as mad cow disease. Scientists are concerned that with space travel, earth may contaminate life on Mars along with the possibility that Mars as well may contaminate earth, a term known as "back contamination". This article then follows up with the emphasis of how hard it is to isolate a disease once it is on earth, referencing back to mad cow disease and its origins: Great Britain.
http://www.space.com/missionlaunches/missions/mars_contamination_010404.html