It's different viruses causing different things. That's why it's so scary. But if something is being transferred by the air, it's very hard to avoid it. Not very long. Less than an hour.
Viruses are strange creations.
They have to live inside a cell. They don't work. Back in 1918 many cities passed laws that you couldn't go out without gauze over your mouth. It was useless. The virus passed right through.