Plague Maths: Plague A + Plague B = Plague A?

It's also just occurred to me that we could have effectively cancelled out our plague of flies by using the plague of spiders to eat them, although that may have entailed chasing the flies upstairs into the bedroom which is where the spiders mainly hang out. Which got me to thinking about plague maths and if other plagues would cancel out or if they'd reinforce each other. Now as previously discussed we could cancel out a plague of birds with a plague of cats, cats with dogs, dogs with cows and so on.

You could argue that although the plague of spiders would eat the plague of flies, it would become much larger as a result and you'd just end up with twice as many spiders, but I'm not sure that would matter much. After all, once you've got to the point where it's a plague how much more plaguey can it get? The answer is none. None more plaguey. Plagues are like infinities in that respect.

Looking at the ten classic plagues you could for example cancel the plague of blood with a plague of vampires. That's probably not a very good example. Let's see... No, I'm not getting much inspiration. I suppose a plague of darkness could be cancelled out by a plague of street lamps.

Yeah, OK, I didn't really think this whole plague maths thing through.