That is an excellent way of thinking about them.

For anyone unfamiliar, OP is referring to the Kurt Vonnegut novel which has a fictional state of water (ice-nine) that immediately freezes all other water it comes into contact with into more ice-nine.

Edit: I committed a sin and didn't mention the title: Cat's Cradle.

There must be some kind of counter-agent, or life as we know it would not exist.

To prion diseases? Mainly in that they're not that common, more like a weird aberration than something that actively fights and adapts for its own survival like bacteria and viruses.

I mean cancer is not contagious, but it still kills millions a year.

Strictly speaking, contagious cancer is a thing, but not in humans. See https://en.wikipedia.org/wiki/Canine_transmissible_venereal_...

Yet.

Not necessarily in a satirical science fiction.

The counter-agent is that it's fictional. ;-) (We hope.)

Similar to that is the concept of false vacuum.

You should read about strange matter/strangelets:

https://en.wikipedia.org/wiki/Strangelet#Dangers