Internal parasites.
It could be a type of worm, but as I've said, there are literally hundreds of potential organisms that cause this kind of wasting disease that's referred to as 'internal parasites'. Worms, protozoans, probably some kinds of amoeba even. Luckily most of them are all sensitive to the same thing.
Guppies are just about the only fish I've had problems with in terms of IPs. Even other livebearers I've kept seem quite resistant to it; I've only ever had one swordtail catch it off a guppy. Paradoxically, all the tetras and rasboras I've kept are absolutely bulletproof to disease in general. I've only ever had 3 out of the... I don't know, hundred I've kept at some point or another die. From my experience, guppies are severely, perhaps even critically inbred and have absolutely zero immune systems. I've argued with a well-known breeder and IFGA winner about this before; you can select against physical deformities but are you ever going to be able to select for immune systems and internal organ function? Your average purebred guppy is about a million generations inbred by now, and that surely has some severe consequences in terms of the efficiency of its internal organs and immune system. Most purebred female guppies can barely give birth without suffering severe injury or death these days.
Do you know what you’re going to do with the fry yet? There’s gonna be a bunch of the little guys.
I'm going to raise them and then sell them