Good questions. 

A) its been 3 weeks since the outbreak of the infection. 
B) there are 2 different types of zombies: [i]runners[/i] & [i]clickers[/i].

[i]Runners[/i]: The usual hollywood-style Z's made popular by movies such as 28 Days/Weeks Later; Dawn Of The Dead; I Am Legend. They can transfer the infection through bites/scratches. A shot to the head will put them down.

[i]Clickers[/i]: Inspired by the zombies in the videogame The Last Of Us. These zombies are blind and therefore use echolocation to navigate/hunt. That being said, silence is the key to working around these guys without confrontation. They too can transfer the infection through bites/scratches. A few shots to the head will usually put them down.