I’m not sure it’s the only factor, but it certainly is one that can be avoided.
A member of staff at my daughter’s school went on holiday to Spain and returned the day before school started. The head had warned that anyone who went to a country where you needed to quarantine on return you must give yourself enough time before school opened or maybe lose pay.
So this upstanding member of the community chose not to tell the school she had been on holiday.
Three days after return she goes off sick with, guess what? How many will she have infected in school? It was completely avoidable but her selfish actions could put someone else at risk.