analog

People often get "pink eye" after having been sick with strep and/or some kind of gastrointestinal infection which causes vomiting/diarrhea because viral "pink eye" is caused by the same virus as strep throat, so all it would take is coughing into your hand and then wiping your eyes... and there are many bacteria that can cause "pink eye", several of which are already commonly found in your bowel, let alone when you're actively sick. This is generally the cause of "pink eye" in children, as they do not wash their hands properly (or at all) after using the bathroom and then transmit bacteria from fecal matter into their eye.

There are vast quantities of research that prove hand-washing between patients in health care settings such as hospitals and doctor's offices cut the transmission of infection drastically. The CDC has identified hand washing as the single most important means of preventing the spread of infection. If you wash frequently, and effectively, you are not causing "super" anything, because they are being destroyed. Also, the mutation of bacteria and viruses is chiefly found to occur in people taking antibiotics. The bacteria is exposed and enough of it is killed off so as to render you "healthy" again, but it's not gone. This is where it mutates and you pass it to someone else. Then, THEY take an antibiotic and the bacteria is, again, fought off but not completely gone, and once again learns to fight the antibiotic.

This of course happens on a large scale over time, not immediately from person to person, but occurs because people take antibiotics and don't use the full regimen, which allows enough of the bacteria to stay alive and learn to adapt. The other reason this happens is because antibiotics are prescribed like candy to everyone who has the sniffles and demands a course of antibiotics. Most doctors are also more than happy to placate people by tossing a Z-pack at them, without testing to see if they've even got a bacterial or viral infection. It used to be that they'd at least attempt to rule out definitive signs of viral infection... now they take a culture to test for bacteria and give them an antibiotic anyway. I know, I see it all the time first-hand.

Also, per the "move" scenario, changes in things such as average temperature, altitude, and humidity can have a large impact on you and your immune system (though generally just at first), as well.

While I agree that you should NOT sterilize your world, basic preventative measures like frequent and correct hand washing go a long way to help prevent the spread of disease. A large portion of this world is covered with nasty stuff because people are terrible at hand washing. People, in general, don't do it nearly as much as they should, if they do it at all.