a mess

Dirk gets his neck scar because when the Striders were grouped up in the beginning with a few strangers they got attacked by a small horde. Hal got scratched across his back via a hunter, but due to the Striders being some of the people that are immune, he was fine after they patched up the injury. The group wanted to abandon Hal but Dave wouldn't let them, having mostly convince them to not do anything unless Hal showed signs of being sick. If he did Dave would and his sons would leave.

Despite showing no signs of illness someone from the group was overly paranoid, most likely infected as well but probably not due to Hal, and tried to murder Hal in his sleep. They fucked up though because instead of the partially deaf brother that they plan to do in, they had picked the wrong twin and dirk woke up in time to defend himself and yell out for help. Dave stops the person but not before they get a slice across Dirk's neck. Dirk is fine, got the skin and didn't actually pierce his windpipe of damage his arteries.

Dirk couldn't talk for a while, up to this point he's fully healed with no damage to his voice but chooses to stay mostly quiet.

And Hal wears the bandana around his neck and sleeps with it covered for Dirk as a way to show his neck is somewhat protected.