The problems are bigger than that. Theres no easy source of issue. For instance, we are overusing antibiotics. There is no quesiton there. But our bigger problems are with viral agents. And of course, prions. And good luck trying to explain to someone why Archaea are different from Bacteria.
Theres also things people dont even realise theyre doing. Like, anti-bacterial hand gel. That is one of the worst ideas (at a consumer level) ever. Anything it doesnt kill is now immune to the damn gel and has no competition in its enviornment -- your skin. Have you been to a doctor in recent years? The last one i was at had "do not ask for antibiotics for your viral disease" posters. There are actually posters about this. Why a doctor cant just tell someone "no" is beyond me but...
And we arent actively changing anything, I dont even know what you mean by that. When I got my (thankfully not antibiotic resistant) staph infection a year ago, no one told me it wasnt a big deal. But then, 40 skin ulcers (there were more; thats how many scars I have from them. not all formed scars.) in a 24 year old tends to be a big deal.
So yes, these things will occur. And it will be bad. But will it be "300,000,000 dead in less than a century" bad, as is the case with smallpox? No. You know we have destroyed two viral agents thanks to our efforts. Thats two more than ever went away on their own...even if one is rinderpest.
We either fight or die. And in a world with less than a days travel to anywhere on the globe, "Die" is easy.