Hello
I have no piggies now but DD and I had a herd of the little demons over eight years .
I used to treat mine with Xeno 450 drops -(buy online ) they need to be over 12 weeks but there is one for piglets too .
It goes on the neck where they cant reach ,onto the skin. Contains Ivermectin which is the active agent for mite treatment .
I did have a piggie with Running Lice but they aren't blood suckers so Ivermectin doesn't work on them, but a couple of washes with Goregeous Guineas Lice'n'Easy ( I don't work for them 
) for all my pigs sorted it (and throwing out the affected hay and all their bedding)
The rescues I got my pigs from used to bath and treat them when they recieved them as a preventative and carrying on the Xeno 450 seemed to work for mine
Most guinea-pigs carry a mite load (urgh ) but don't cause any problems . It's if they're ill or stressed they can take hold and you'll notice typically balding snapped hairs on the rump, intense itching and scratching and an unhappy pig. 
. They can die from a heavy infecstation!
Our smooths and abby had a greasy spot on their necks for a few days , our Rex and Teddy boars absorbed it leaving no trace so they could pretend we didn't care enough to treat them
So..........you might want to disapper down the rabbit hole of threads on here about guinea-pig food/ guinea-pig bedding/ guinea-pig First Aid boxes ( lots of things are handy to have , when guinea-pigs get ill they do it quickly ) , Hot weather/ Cold weather care .
Lots on here .
And Pigtures
The currency for advice on Small Pets is a photo of aforementioned guineas 