I think they actually do understand but don’t have enough empathy with other people to see it as their responsibility to protect other people from their viruses.
Not that someone as perfect as them would ever sick enough to potentially infect others…
This. So much of this. I can’t even convince family members to not go and socialize with dozens of others while they are sick! Five years ago, I would have bet my life’s savings and every appendage I have that I would get the correct answer if I asked someone whether illnesses spread through contact with or being near a sick person.