How can people living in a house with an infected person test negative?
Have you ever lived in the house with someone who had the flu and didn't get the flu? With someone who had a cold and didn't get the cold? Happens all the time. COVID is highly contagious but it's not on a different order of magnitude than a lot of respiratory diseases we already experience. It's just the combination of contagiousness, fatality rate, and having a new introduction to the population all at once that's causing the problem.
Any of the following are valid answers to your question in different cases:
1. Maybe you got the virus from them, but had a highly effective immune response that defeated the virus before you went to get tested.
2. Maybe you already had the virus earlier from someone else and are immune now.
3. Maybe you already had a related but less deadly coronovirus in the past, which filled your body with antibodies that kept this new coronovirus from taking hold.
4. Maybe you took appropriate precautions (not kissing, not drinking from the same glass/bottle, keeping coughs/sneezes covered, not spending extensive time in close proximity, washing hands before putting them in your mouth, etc.) so you weren't getting hit by a high viral dose.
5. Maybe you are infected, but you don't have a high viral load in your nostril or there was a glitch in the testing and you got a false negative.
6. Maybe some other answer we don't know yet. We've only known about this virus for a few months, there's lots of shyt we don't know. It took YEARS to figure out AIDS and we're still figuring out shyt about it.
Back at the end of July my foster daughter got sick, then I got sick. My wife and my baby daughter never got sick. We're not sure if it was corona or not, it was never bad enough for us to go to the hospital. But regardless of what it was, obviously we had some sort of virus in the house and it only hit my foster daughter and me even though literally everyone in my family spends more time with my foster daughter than I do. My wife always mocks me that she has a stronger immune system than me, maybe she's right.