The virus is excreted in poo, so if you flush without closing the lid, it aerosolises and will land on surfaces in the room. It can (in theory) then be picked up by other people who touch surfaces in the loo and then touch their mouth, nose or eyes.
OTOH - don’t you wash your hands every time you go to the loo, and extra thoroughly when you’ve done a poo? So the risk of infection is from touching other surfaces after you have washed your hands, e.g. the tap or door handle.
So, the procedure needs to be:-
- do your wee (or worse)
- use left hand to put lid down, flush and turn on tap (this assumes you have used your right hand to wipe)
- while washing your hands, also wash the tap you touched and door handle. Ordinary hand soap kills the virus
- Turn off the tap, dry hands, ideally with disposable towel.
- Use tissue to open door without touching handle.
- Cleanse hands with antibacterial after leaving loo.
There are several redundant layers of protection in there. And even if your host hasn’t provided sanitiser and disposable towels, you can take your own.
In the event that your would-be host can’t cope with the above, the pop in for an hour approach would be what I’d do.
When we had garden visitors the first time it was allowed, we thoroughly cleaned the downstairs loo before they arrived and put a clean towel in. While they were here, we used the upstairs loo. After they’d gone, we cleaned the downstairs loo again, sanitised potential touch points between the downstairs loo and back door, and put the towel straight in the wash. But they were from one household, so slightly different situation.