Coronavirus FAQ: My partner/roommate/kid got COVID. And I didn't. How come?

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As a physician and infectious disease epidemiologist, I've seen a lot of COVID-19 patients during the course of the pandemic, and there's a question I hear over and over:

How it is possible that my partner – or child or sibling or roommate – tested positive for COVID., and even though I slept in the same room or lived in the same house, I didn't come down with the virus?

Weren't they breathing out infectious particles for days on end? And I assume I was breathing them in.

There is an answer to this question. But it's a bit complicated.

First: Let's review how SARS-CoV-2, the virus that causes through the air in invisible aerosols (and to a lesser extent in large droplets) that the infected person emits while breathing, talking, sneezing, coughing, laughing or snoring. The aerosols can hang around in the air for hours, and others can inhale them.