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They're like an unholy hybrid of vampires, zombies and Spider-Man.
More plot-holes regarding I Am Legend, and I'm going to spoil it for you...
When the vampires (like I said, dark-seeker sounds stupid) break into his basement lab, Will hides the woman and child into a small compartment at the very back and tells them to "wait for dawn". Why does this matter since no sunlight will be able to get into the underground lab anyway, meaning the vampires can stay down there indefinitely and therefore can reach and kill the woman and kid easily. Also, Will's contingency plan for when the vampires manage to break into his home is very ill thought out. Hide in the basement (where there is no way out, which he really should've prepared for) and when cornered by a room full of vampires, his escape plan is...
...a single hand grenade that has been kept in a drawer full of medical equipment, which he uses to sacrifice himself by throwing himself at the vampires when it explodes. This doesn't make any difference to the woman and kid because since all the vampires know where they are, wiping out a dozen of them means nothing when the rest come along. So they couldn't have survived to find the people at the end.
As I said earlier, it's easier to believe the whole thing's a dream Will has as he dies at the hands of the vampires.