A toilet cistern tends to have 2-3 gallons of water, which would translate to 20-30 portions ingame, but the water in the cistern is isolated from the rest of the system, it’s generally potable if the inside looks clean and the tap water is potable. At worst, it would have a stagnant taste, and that’s after a very long time of not being used.
I’m fairly certain that the reason for poisoning isn’t that you are drinking out of the bowl, but rather that there are a large amount of newish mutant bacteria getting into the water supply and nobody is manning the water treatment plants to remove them. That’s why any type of water other then clean water (even rainwater collected with funnels!) has a chance of poisoning a person, though that chance varies depending on where the water was gathered.
Assume all toilets are like bar toilets.
i.e the last user left a floater in and didn’t bother to flush.
You better damn well boil that water.
Yes, my understanding is that all surface water has been contaminated by the same junk that caused the cataclysm in the first place. And the pumps aren’t working to get clean well water. Or something. I never was sure why toilets have an infinite supply of water (they should only contain about 6 bottles worth of water).
The Blob is in the groundwater, and anyone who used it for about a week or so pre-Cataclysm is probably infected. The reason Science or Military corpses stay corpses is presumably because they were operating on bottled water.