"The most dangerous and rare cases, these are the recently Infected who still possess much of their strength and coordination. They move very fast and are very strong."
Runners are slightly more dangerous than most type of infected, but are still typically relatively easy kills for a well prepared player. But be cautious if there are several; unlike Walkers, the 'Fall Back' option will not cause them to merge into a Horde, so they must still be killed individually.
One place where numerous runners can be encountered is the inside of Theater 1 Encounter.