captaincrapster:

The Striders were employed on a nearby base prior to the outbreak. It was assumed to be biological warfare, though only military personnel and those working closely with intel can claim to know the strain’s origin. 

Dave was discharged for reasons yet to be revealed. Bro is still taking part in active duty and has been identified as a carrier of a gene sequence that is likely to cause dormant infections to evolve into fully-rendered zombie funtimes. 

Despite the likelihood that Dave has it too, he refuses to get himself checked out.