Measles are still around, killing tens of thousands each year. Which is significantly less than the millions it killed before the widespread availability of the vaccine. Unfortunately, vaccination rates are dropping in many places, partly thanks to folks like Flipnile going "the vaccine might be worse, we don't KNOW" (we do, it's not).

Smallpox has actually been eradicated, but they keep a few samples locked away for research purposes. Apparently there's controversy over whether this is actually needed, with Russia and the US flouting recommendations by the WHO (what a surprise) to completely destroy the last remaining samples.

As far as I can tell, nothing has taken the place of smallpox in the natural order of things. I wonder what exactly your professor was talking about and how well you remember it, because I'm not familiar with this theory that eradicated viruses leave a vacuum that will necessarily be filled with another dangerous thing. If you have more specific information, I'm interested to see it. I'm not a scientist, so it's very possible that I've simply never read about this phenomenon.

Finally, and I hope anyone feels free to correct me if I'm wrong here, I'm pretty sure that viruses only replicate inside a host, so they won't evolve in response to selective pressure outside hosts, which means that they won't develop resistance to detergents (which I think work by destroying cell barriers, something which is hard to evolve against anyways, just like humans won't likely develop skin resistant to acid or fire).

Bacteria don't need a host to procreate, so they might be able to develop resistance against things designed to kill them outside a live host (so not antibiotics). But I'm not sure whether detergents etc. count, because as I mentioned above, developing a resistance against soap/alcohol/etc. dissolving the cell barrier isn't very likely to happen because a few genes change.

If there are any biologists in the house, I hope they'll take this opportunity to shoot holes in my patchy knowledge of these things. I tried verifying what I remember, but I couldn't find specific info on evolving against cleaning supplies in the time I had. Which could mean anything, lol.