in most cases, guppy's unfortunately contract a disease called "tetrahymena" PLEASE CHECK THE SPELLING sometimes called saddleback in the trade. Its a bacterial disease which is very hard to cure , and usually very catching to other tank mates.
theres only one treatment im aware of myself on the commercial market, called "chloromin t" its made by a koi make called kusuri and i think its quite expensive but its the only thing ive found to control or cure if used quick enough.
please don't take this as the definate disease, because its very hard to see on the pictures but 9 time out of 10 its this. Other symptoms consist of a dosile action at the top or bottom, with the last half of the fishes body hanging or drooping lower than the head causing a perculiar swimming action.
just for future reference there are guppys which have been supposedly vaccinated against saddle back, usually comming from czech or israiel.
hope this is of some help, goodluck mate.