"This infected was a worker from some colony that was infected in the past."
These infected came from an old colony which was recently infected. This infected has 45 hp and can run much faster than the standard aged or decrepit infected.
These infected can spawn from standard infected hordes and mini hordes, Doom Villages and around the map as well. These infected also spawn if a colony building becomes infected.
If neglected these zombies can easily infect some of your buildings so be aware.