…because fecal matter and vomit can become aerosolized and molecules, including those carrying virus cells from flu or norovirus (better known as the winter vomiting bug), and these airborne cells can be breathed in or land in food or drink, contaminating a new person. Airborne fecal or vomit cells are a known viral transmission method.
Eeeewwww.
So close the lid whenever you flush and keep food and drink out of a bathroom, especially when a family member is already suffering from a virus! Store any bathroom cups upside down to ensure they stay clean, and utilize the fan after stinky sessions to remove any stray airborne molecules.