Its NOT WASHING YOUR HANDS AFTER YOU GO TO THE BATHROOM AND BEFORE YOU EAT !
Went on a cruise with the wife in January, her first, my 2nd in 40 years.
At the entrance to each restaurant and buffet were washing stations with attendants saying “washy washy before yummy yummy! Most people did wash their hands. Didn’t get sick and the food was really good, but stuck on the ship for 5 days was a bit confining.