http://thehill.com/op-eds/missing-out-on-outer-space-2007-06-12.html

"If you send astronauts or you send samples for return missions to Mars and you bring materials back from Mars, you could contaminate the Earth with Martians!
If you plan to send human beings to Mars, which is the plan of Europe and the US several decades from now, you must be sure that they come back alive and not contaminated by strange organisms or diseases. So you must make sure that Mars is a completely sterile planet and that there is no danger of contamination.

"If it is not a sterile planet, humans must analyze the forms of life which may have been discovered there, and determine whether or not they are dangerous for humankind. "

Meeting this standard will take considerable time and money. In the meantime, a tremendous planetary defense system can be constructed and put into operation.