I don't think this is known or knowable at the moment. Hence the worry.
You are right that airports are probably a greater risk. Perhaps they should be closed. On the other hand, that would be far more disruptive.
As for basic hygiene, yes, we all should. At the same time, we all know that a significant fraction of the population can't or won't do this.
Edit: Here's the proper method: https://www.who.int/gpsc/clean_hands_protection/en/
A significant caveat is that the sink/restroom must allow proper handwashing. Many lack paper towels, for example. If you have to touch the faucet handle or the door after you start washing (which takes two minutes), the method has not been followed. It's harder than it looks.
[1]: https://gisanddata.maps.arcgis.com/apps/opsdashboard/index.h...
It only takes one person leaving without washing their hands for the irony to be apparent.
Can any of them come infected by the COVD-19?
The probability is very low. Almost all of the participants will come from areas where the presence of the COVD-19 is nil or extremely low.
By comparison, a number of passengers equivalent to three times the MWC go through the airports of Atlanta or Los Angeles, two times pass through the airports of London, Paris or Amsterdam each day, and 1.5 times through Madrid and Barcelona each day. And none of them closed so far.
The basic hygiene measures (handwashing, not sneezing or coughing to the open) and common sense are more solid and effective.
If we follow the same line of thinking (we shouldn't), we would have to close all the airports tomorrow.
edit: typo