Yes the bug bit the son. But was it the bug that caused his teeth to fall out, or was it the radiowaves like in Cuba as KB said? We don't know, they don't know. No-one will ever know. Exactly like it could happen IRL.
I thought the first KB scene was good. They rocked up assuming he'd help. In a real war/breakdown of society there's no flags saying "ok guys, we've now hit the point where guns do the decision-making and money doesn't matter anymore. And that neighbour you thought was a friend, actually isn't anymore". That's literally the point. I can see interactions like that happening IRL if some big event like that happens. You think when there's a natural disaster like Katrina or something, there's some sort of broadcast that says "ok guys, it's all gone to hell so we've hit the point now where you can go and loot and shoot people up". No, these things are all grey areas and they happen rather naturally.
This sort of thinking from the filmmakers was flagged at the beginning when GH turns up saying it's his house. Bullock is thinking, but doesn't want to say that she doesn't believe him - because he's black. It's a very real-world interaction, lots of grey areas. I thought it was all well done.