The Bridge, by Skipp and Spector. Been awhile since I read it, and I’m sure it’s definitely dated by today’s standards, but it creeped me out but good anyway and was actually the first thing that sprang to mind upon seeing the title of this thread. Basically, a badly polluted area mutates and goes on the warpath.
From Amazon:
“Nature evolves according to its environment and the changes in its ecosystem. Poison it too much, and it will compensate in the most horrific way possible.”
Also one thing I remember being fun about this book was the list of music provided that the authors listened to while writing it.
I found Floating Dragon, by Peter Straub to be quite a thrilling read. My limited knowledge of matters scientific was insufficient to poke any holes in the plot. Exceptit didn’t make sense that people infected with the bug dissolved so badly that they had to wrap themselves up in Ace bandages to not literally fall to pieces; yet, with the wrappings, they were still functional.
If you want a macroscopic critter, rather than a disease, my pick is Niven and (somebody… Barnes, I think?)'s Legacy of Heorot. I’ve never heard of any non-sentient SF beastie nastier than an Avalonian grendel.
The sequel, Beowulf’s Children, is also good, but by that time, the humans have learned how to effectively fight and avoid grendels, so the plot ends up being more man vs. man than man vs. nature. Though there is still a bit of the latter, in the speedfly swarms.
In Asimov’s first novel, Pebble in the Sky (one of my favorites, despite its many flaws), the terrorists’ superweapon is an engineered plague designed to kill everyone but themselves. But the plague itself isn’t actually too significant in the story, beyond being a plot-driver.
And since “The Last Flight of Dr. Ain” and “The Screwfly Solution” have already been mentioned, I’ll also bring up “Houston, Houston, Do you Read?”, by the same author. The disease has already run its course by the time of the story, and killed everyone vulnerable to it (about half the population), but the world has been changed dramatically as a result. The story mostly deals with a group of Rip Van Winkle-esque astronauts slowly learning about the changes.
Oryx and Crake by Margaret Atwood where the antagonist is biological engineering and how it has basically wiped out mankind. The book starts with mankind being essentially gone, and backtracks to how it took place.
I highly recommend George R.R. Martin’s collection of SF short stories Tuf Voyaging, about the master of an ancient and very powerful starship well-equipped for biowarfare. He uses its capabilities, or threatens to, in some very… interesting ways.
A great book - I re-read every few years, and get something new out of it every time.