https://www.who.int/tb/areas-of-work/drug-resistant-tb/xdr-t...
What if it becomes common enough to get into the water at much lower doses? What if people split pills to reduce costs? ISTM that there are many potential avenues for evolving resistance.
No, the first bug chaser to successfully contract HIV while on it was quite proud of his achievement. Clearly the virus can survive.
If someone is using PrEP and is exposed to HIV, then either the drug works and they avoid infection (in which case there is obviously no impact on drug resistance) or they become HIV+. But in that case, they were either exposed to an already-resistant variant, or they were astonishingly unlucky and the drug failed to prevent infection - in which case, resistance is a moot point.
A person needs to develop an active HIV infection in order to communicate the disease to another. PrEP prevents this from happening in the first place.
That's why it's important for people taking PrEP to be screened regularly, and put on a proper cocktail if they test positive.
https://en.wikipedia.org/wiki/Antibiotic_prophylaxis
(I'm talking about the use in humans there)