http://abcnews.go.com/sections/science/DailyNews/oberg990204.html
To quote in part:
In a 1994 article entitled “Is It Dangerous To Return
Samples From Mars To Earth?,” Carl Sagan wrote: “If
putative Martian organisms were originally transferred to
Mars by collisions with the Earth, they may be enough like
us that they could be pathogenic.”
The same argument holds if the transfer was from
Mars to Earth.