How does the dog in i am legend get infected and dies??

I just saw I Am Legend today and that was the saddest part. Will Smith and Sam (the dog) get attacked by a couple of the mutant dogs. Will is ok because he is immune to the virus but Sam is badly hurt. Will takes Sam back to his lab to try and save her with a cure he's been working on. The cure doesn't work and she begins to turn. He has no choice but to kill her and he does it with tears in his eyes.
There were a couple of dudes sitting next to me and I could hear one of them sniffling. Hell, I'm starting to tear up a little just thinking about it.

I boo-hoo'ed like a baby. I thought I would have to leave the theatre. But, I am sure that I heard Neville report in his daily log that 'he was immune to airbore and blood contact of the virus and canines only were immune to airborne.' He was not sure why. Report It

i don't no how it got infected but i no will smith chokes the dog

When Sam the German Shepard saved Will Smith after he stepped into the booby trap. She got pretty badly mauled by one of the mutant dogs. Earlier in the movie, when Smith was recording notes he stated that canines were immune to the airborne virus, but not to the contact virus.

the other dogs that are infected bite him. even though will smith tries to cure him, the dog contracts the virus and tries to attack his owner, so will just chokes him. :(

Two infeected dogs get it. WILL SMITH TRIED TO HELP, but he was injured.

the alpha-monster guy releases dogs when will gets caught in the snare. then one of the dogs was biting sam in the throat.