I think people understand the basics of COVID-19 well. What they don't realize are how many "sick" people are around them. See also the health care debate over "pre-existing" conditions. Until you have one, or have a family member with one, you're pretty clueless.

Autoimmune diseases are under diagnosed, but some estimates say it's more than 1% of the population. That's roughly 3.3 million Americans that may be taking medications that reduce the efficacy of their immune system.

TLDR; Don't assume everyone is healthy / carries the same risk, even if they look healthy.

Edit: Fixed my math.