If this thing triples in 3 days and you infect 1, 2 or 3 people, and then each of those do, and then each of those do, and then each of those do, then you've got a very large unsurveilled group in a locale, very quickly. This is a pathogen for which there is no natural resistance for.

Some portion of that group would visit a clinic or ER, and as we know from China, even trained medical personnel went down with this, quickly. It kills at about 10 times the rate than flu. I find it difficult to believe that in every country we know people have been traveling to, from Wuhan, that there are a lot of opportunities for this to absolutely decimate healthcare staff, even in one hospital or whatever.