Anyone who’s been on a cruise ship knows how strict the hygiene standards are due to the speed with which an illness can spread amongst hundreds of people in a confined area for an extended period of time.
An airliner can be worse as people are much closer together and breathing recirculated air. Toilets have a heavy demand on them as each passenger makes multiple visits during a long flight. Aircraft routinely visits places with poor hygiene standards and endemic diseases.
I’m surprised incidents like this don’t occur more often.