Except, strangely enough, according to my anecdata, not COVID-19. For nearly 2 years there were, AFAICT, almost no cases of COVID transmission through our nursery. I'd love this to be followed up properly. I have a theory that not only are the kids more resistant, but the parents are too due to the circulation of the other coronaviruses in pre-school children, along with all the other various infections.
It's all changed somewhat with Omicron, but then it has everywhere.