When my children were in primary school one of the dinner ladies, returned from abroad and brought Shigella into the school. The infection was passed on because she prepared food whilst still infectious.
I'd never heard of Shigella before this but it was, and is, a notifiable disease and is pretty unpleasant.
It ran rife through the school and through many of the families in the school, many of them being very unwell.
One of my children caught it and was very unwell. I kept the other children home from school and she was the only one in our family who got it.
The reason no one else got it was because I was very strict about hand hygiene. No one else was allowed to use the lavatory she was using, but even so I swabbed it down every time she used it and we all washed our hands thoroughly after every visit to the lavatory, as we all did anyway.
I also always washed my hands before preparing food and everyone washed their hands before eating.
When you use a lavatory and don't wash your hands, particularly a public lavatory you have no idea what you might been picking up from the person who visited it before you. You then, quite likely, take it with you into your home.