Or click on the GuineaLynx button above. You might want to read about mites there and check back with further questions. This a topic that many guinea pig owners encounter and I´m sure someone here will be able to answer your questions.
Ivermectin is the treatment of choice. The sooner you take care of it, the better. Healthy pigs are sometimes not as prone to severe outbreaks but if you are experiencing any of the symptoms you describe, it´s a good bet they need an ivermectin treatment. You can read about ivermectin at the same site (look for "Ivermectin" in the index).
If it is a very flaky spot, it might be fungus. I have a sow who seems to constantly have fungus. Some home cures for fungus are athletes´ foot spray or cream after a bath ( I use selsun blue, but others use nutrogena t-gel and other human dandruff shampoos to battle the fungus), a series of baths in an iodine based shampoo, or reppeated application of betadine or another iodine based formula to the affected spots.
You can also get something that is very effective against fungs from your vet, it is a dip called Lym Dyp. It stinks, and is an ugly yellow, but it works.
No matter how you treat fungus you will have to keep up the treatments for a week to a month, as well as cleaning the cage thouroughly to get rid of the problem.
If so, lice can easily be treated with Advantage, a topical anti-flea medication for dogs and cats. You will need just ONE drop on the bald spots behind the pigs ears.
If you are not sure exactly what the creatures are, it would be best to take your pig to see a cavy knowledgable vet. Preferable one who specializes in exotics rather just cats and dogs.