I've seen comments here trying to play down the message or discredit the signatories of this letter. Don't. Please take it seriously. This is not sensationalism. What the letter means is that if your country is not there yet, you might be DAYS away from widespread panic and total lock down. I'm writing this from Barcelona, Spain. This last Sunday, 6 days ago, I went on a walk with my girlfriend, had a nice lunch, too…
I fear that the situation is already at that state in the USA. There are 1600 confirmed cases but the lack of tests hides how many real infections there are.
The fact that many people here and around the web are sharing stories of sick friends is are a set of anecdotes that would be nearly impossible if the reported numbers were correct.
I suspect by this coming weekend the US will feel very different. I wouldn't be surprised at all if a major update to official numbers came and completely changed the current state of where things are.