Tooth and Nail

Tooth and Nail is a 2010 horror novel by Craig Dilouie. It follows the struggles of Charlie Company, a group of army soldiers deployed in New York to keep the peace during an escalating pandemic. Things start bad--there's no cure or vaccine for the virus, services are breaking down, and people are angry--and quickly get worse.

Not to be confuse with the indie fantasy, action strategy indie game that has the same title.

Tropes used in Tooth and Nail include:

Apocalypse How: At least a Class 2, potentially a Class 3A or even Class 4. It's made explicitly clear that if a vaccine was developed today, it would be months until it was mass-produced in large enough quantities to effectively stop the spread of HK Lyssa, which means that so many people are going to die off before the pandemic ends that there's not going to be much in the way of a functioning society left. It's not outside the realm of possibility that it could be a Class 3a species-ender; and since the Mad Dog strain is based on rabies, and rabies can affect any mammal, there could be a catastrophic impact on the biosphere.
Ate His Gun: Private Chen, after the shootout at Tilden Middle School.
It Got Worse: Every time Second Platoon move to a new location, they end up in a worse position.
Not Using the Z Word: Mostly averted; individual Mad Dogs are never specifically referred to as zombies, there are several moments when characters say that the infected are acting like zombies.
The Virus: Hong Kong Lyssa, which is transmittable by air and carries severe flu-like symptoms with a three to five percent mortality rate, and in rare cases will enter the brain, causing dementia and aggression. Then there's the Mad Dog strain, which is transmittable by saliva and turns people psychotically violent, has 100% infection and mortality rates, and an incubation period of as little as an hour.