I use the apocalypse movie threshold.
If you start to look like the last person on Earth in a world with toxic air/radiation, either:
1) You're being too cautious.
2) You're going to get it anyway.
Stay away from people, wash your hands, wash your surfaces. These are all reasonable things people can do to fight the unpredictable, invisible enemy.
Think about my apartment. There are about 15 different units.
Imagine if I walked outside and I was the only one. I didn't touch anything, and no one came out there with me.
Now, if it stays in the air, I could be exposed to it right then and there. Then I walk upstairs to get into my car. It's entirely possible that particles from an infected person are already gathered all over the door handle.
And what of my steering wheel? I'm the only person who has ever touched that, and I watch my hands constantly.
And where am I going? To the grocery store to pick up stuff for my mother to survive. Keep in mind we're not actually interacting.
So I leave the groceries outside her apartment door so we don't chance breathing on each other, and every fear I just had about merely existing in the world is compounded onto her outdoor air space, milk, cereal, frozen dinners, ice cream, soup, etc.
Every time someone gets this they try to survey them and very rarely does someone say "I was just staying at home in quarantine but I made a mistake and picked up an outside object.
If someone can find me such an interview, please let me know. The variables I am seeing is that it has infected and killed people who were perfectly healthy.