External parasites are easily picked up by guineas and passed on to other guineas through close contact. 2 rounds of 2 shampoos with Lice 'n' Easy Shampoo a week apart are required to wash them away. Guinea pig parasites are usually species-specific, so you don't generally need to worry about catching anything nasty from your guinea. They can be difficult to get rid of with just the shampoo, so MicroNeem can be used before the shampoo. Always follow the instructions carefully for the best results.
If you find a guinea with external parasites and it lives in the same cage as others, they will all need bathing at the same time. Always make sure that the guineas you have bathed are warm and dry before putting them back into a clean cage. Don't let them come in contact with the guineas waiting to be bathed. A couple of lice is all it needs to keep the problem going.