Cases in Italy on 22.2.20 = 20
Cases in Italy on 2.3.20 = 2036
That's a rise of x100 in less than 10 days. Now, I think that number is due to uncovering a pre-existing cluster, and I don't think the rate of increase will maintain at anything like that level. According to the best figures available, this virus is supposed to double every 4-5 days. I'm working on the basis of x8 per fortnight, because it's easy to make rough estimations that way.
So, let's look at the UK. We have 40 cases now, and we'll likely have 320 by the end of the fortnight, and roughly 2,500 by the end of the month.
Except, there is a fortnight's incubation, so we probably really have that 320 now, and will really have 2,500 in a fortnight, though we won't see them yet.
These are all just rough calculations, and the risk at this point is still very low. But let's play out the next month in the UK...
6 weeks time: 2500 x 8 = 20,000
8 weeks time: 20,000 x 8 = 160,000
And those 160,000 will already be infected in 6 weeks time and incubating, waiting to be confirmed. Do we still think JUST 40 cases is nothing? Do you want to wait until there are thousands of cases confirmed (and potentially 8 x more that number still incubating) before you pull your children out of school?
As a maths teacher, I will always tell my students, "interpolation is reliable, extrapolation is not." If exponential growth in diseases continued unchecked, we would all be dead of ebola by now. But people are not being idiots for looking at "just" 40 cases and starting to get concerned.