In Doom (2005), On Mars, at a UAC Research Facility, scientists are attacked by monsters which are infected. One of the scientist send video message to Earth. After seeing this message on Earth, a team of eight Marines known as the RRTS went to Mars to kill the monsters.
After killing all the monsters, they returned to UAC facility on Earth. But Team find that all humans there were dead.
How did the people on Earth also get infected?