Who is The Vampires from I Am Legend and what is their importance?

Everyone except Robert has been infected by a plague, which gives its victims the characteristics of vampires. They crave blood, their skin turns pale, they cannot stand to be in the sunlight, and they are repulsed by garlic and crosses. Their cells produce a glue-like substance that surrounds bullets as soon as they enter the body, so that bullets have no effect on them. However, wooden stakes driven into their heart will kill them. During the day, vampires sleep in a comatose state, but arise at dusk to seek more victims.