I've been fiddling with this idea in my head - Once in a while, military "cleansing" brigades will blitz neighborhoods. I was thinking humvees with gunners, from which hazmat-clad soldiers may occasionally disembark, depending on the level of infected, I.e if there are too many, they won't risk it and will leave the work to the turret gunners. Basically, they come in and basically fuck stuff up with automatic weapons of all kinds. Another spin on this idea is that instead of cleansing, the military will occasionally move convoys with either humvees or trucks through. Although by definition not hostile in this scenario, they will fire at anything close to the convoys, survivors or dead-heads with heavy mounted weaponry.