I see a lot of guinea pigs in my practice and the single most common problem I see in them is skin mites. Now first I want to clarify that mites and lice are very different. Mites are microscopic and lice are barely visible. Though they may have some similarities in causing itching, their behavior and progression are different.
A guinea pig with skin mites is invariably itchy. Sometimes mildly so, but often very significantly so. In fact, a guinea pig can itch so severely that they will go into full-blown seizures. This may happen spontaneously or after handling or scratching. Though the seizures are scary to see, they aren't harmful and will go away once the pet is treated.
These pets will start to have scaly, scabbed skin and will lose hair. As the disease progresses the skin symptoms will worsen, sometimes to the point of a virtually hairless guinea pig. Much of the skin may be injured from the pet scratching themselves so deeply because of the itching.
In the early stages it may be difficult to tell whether a guinea pig has skin mites or ringworm (a common skin fungus). Two tests are normally needed. The first is a skin scraping where we try to find the mites under the microscope. However, this is not always fully diagnostic, as the mites may be present but in few enough numbers that they're not easy to find. A good rule-of-thumb that I use is that if the pet is significantly itchy or is having seizures, it's almost assuredly mites since ringworm doesn't cause severe itching. If there is any doubt, a fungal culture of some hair samples is needed. Don't let a vet use a "Wood's lamp" or ultraviolet light on these pets in screening for ringworm. The principle is that certain species of ringworm will fluoresce an apple-green color under ultraviolet light. But guinea pigs and other small pets aren't infected by these species, only by species that won't fluoresce. So though this is a common diagnostic tool in dogs and cats, it's worthless in guinea pigs.
Once a diagnosis is made or significantly suspected, treatment is pretty easy and effective. Most commonly they will receive injections of ivermectin, a common antiparasitic. In most cases a series of two injections spaced 10-14 days apart will resolve the problem, though occasionally a third or even fourth injection might be needed. I've never seen a case that needs more injections that this, so if it's not going away after four injections I'd re-evaluate the case. I also recommend thoroughly cleaning the cage and replacing all of the bedding twice weekly until the problem is resolved.
All-in-all this is a very rewarding condition to treat, as they almost always get better.