The fundamental connection: people who drive SUVs and people who skip flu shots don't care about externalities.
It's not that I don't care about externalities, but that I'm going to weigh them against direct concerns. It's true driving an SUV might have a small percentage impact on the environment. It's also true that it slightly increases my chances of killing someone else in an accident. But if it significantly decreases the chance the person killed in said accident is my wife/kid, I'm going to prioritize that over the "dangers" SUVs provide to others.