Why is it?

Why can you announce there is a virus infecting a city that can kill you and you should avoid contact with people as much as possible because anyone you meet may very well be a carrier. People will ignore it for the most part and go on like it is an average day.
But you put ONE sniper on ONE building with ONE bullet and he will clear the streets for the most part.
Curious as to what kind of answers I will get to this.