Quarantine

Early on March 11, 2008, news reporter and cameraman, Angela Vidal and Scott Percival, are assigned to follow firefighters Jake and Fletcher during their nightshift. They are given a department tour, but an emergency call dispatches them. Arriving, screams from a self-barricaded apartment block room were heard by the landlord and residents. The firemen, police officers, and crew enter; they are attacked by an aggressive elderly woman, who bites a policeman and is then killed. As the residents head safely downstairs, the team finds a second old woman in a similar condition and bring her downstairs with others. Fletcher then mysteriously falls to the base floor, incapacitated.
The residents panic as the authorities and CDC suddenly quarantine the building, allowing none to leave; injured are taken to the woodshop. Meanwhile, Angela interviews an ill, young local named Briana, who states that her dog, Max, is at the vet because he "was" sick as well. A health inspector wearing a hazmat suit arrives and attempts to treat the bitten people, but they thrash violently, forcing the others to flee.
The health inspector reveals that yesterday, a dog was taken to a local veterinarian. The dog became violent and infected the other pets at the clinic, later euthanized; the CDC traced the dog back to the building via collar. The inspector tells the distraught residents that a virus has transformed those infected into bloodthirsty creatures. Angela realizes that the dog was Max. When confronted, Briana snaps and bites her mother, escaping upstairs. The team are forced to handcuff the mother to the stairs to stop her from following Briana. All the other infected also break loose and start attacking. The team retreats upstairs and lock themselves in a room, but discover two people have been bitten. A panicked resident who decides to signal for help by breaking through the window is shot by a police sniper overlooking the building. The landlord reveals that the basement, which connects to the sewers, may be the only way out. The two infected then attack, forcing Jake, Angela, and Scott to flee the room. Everyone else is infected or dead, leaving the three to survive.
Jake is eventually bitten as the trio find the basement key. Angela and Scott now appear to be the only human survivors. Rather than making their way to the basement, the pair are forced upstairs to the attic apartment by the remaining infected. They then search that apartment and discover that its former owner from Boston was a doomsday cult member responsible for breaking into a chemical weapons lab and stealing a virus. As they continue through the apartment, a door opens from the attic and Scott uses the camera light to investigate, only for a boy to swat at it and destroy the light. Scott turns on the night vision and him and Angela hear loud banging noises inside the apartment. When Scott looks around with the camera, he sees a man, who along with the boy seems to have been left in the apartment to rot.
The emaciated man begins searching the kitchen, unaware of the duo's presence. Scott attempts escape but trips and drops the camera. Angela retrieves it and looks around the room, only to see the man eating Scott. In fright, she cries out and is attacked. She drops and is unable to locate the camera; as she is crawling in pain, she is then dragged screaming into the darkness.

Question: What became violen and infected other pets at the veterinary clinic?
Answer: a dog