We're all living there.

You have no epidemiological evidence to blame your housemate.

Perhaps this person had no alternative but it seems a poor choice to move in with strangers during a pandemic. At the very least, the newbies should have to earn your trust over time and perhaps you could have taken a more cautious approach rather than taking their word. That being said, The bigger problem is that you can't confront them in person. Life is going to walk all over you..