Hi!
Skin parasites are species specific; guinea pig mites and lice cannot survive on humans nor can human lice and fleas on guinea pigs. But if just the thought of parasites makes you feel itchy and bad, have a good wash in a shower in order to feel better in yourself.
Skin parasites are unfortunately a matter of fact in any pet. As long as they get treated promptly, they will not get past nuisance level and are not a big problem.
Guinea pigs can have two kinds of mites, potentially lethal mange mites (trixacarus caviae) that burrow their eggs into the increasingly inflamed and painful skin and hay/fur mites (chirodiscoides caviae) which live on hair debris and fix their egg cases onto the hairs, especially at the bum end; they are the least harmful skin parasite and more of a nuisance than anything. I assume from your comment that your vet has diagnosed the latter?
Treatment for both varieties of mites is the same. It is course of 3 applications of ivermectin or selamectin to the skin at the product specific interval. Just a single application is not enough; it kills the current mites, but not any emerging mites - that is what the two follow-up rounds do in order to catch any stragglers and prevent a return.
The active ingredient in Revolution is selamectin; my own spiggy savvy vets treat hay mites with the correpsonding selamectin UK brand Stronghold, so your vet is doing the right thing.
Hay mites usually come with hay; often with shop bought imported pet hay. We have seen a surge in hay mite cases in recent years since the largest UK chain started to import timothy hay from the US. These hay are also more resistent and can be harder to get rid of than the hay mites that used to make an appearance each autumn in cheap untreated farm/equine hay. In persistent cases treating with a vet approved lice shampoo in combination to ivermectin or selamectin can do the trick. You have to wait 48 hours between any treatment on the skin (whether that is a spot-on or a shampoo) before you can apply something else to allow the previous application to be absorbed fully and not be washed away.
I would give your piggy cage and bedding a deep clean but strongly recommend to get rid of your hay and switch the hay brand.
Omg i usually get kaytee timothy hay but ran out and chewy was taking forever to deliver.. i purchased target hay for one night.. tossed when my shipment arrived ... could that be where they came from? I thought they cant live off pigg long!
More information on guinea pig parasites:
Guinea Lynx :: Parasites