There are two main ways that your rabbit can contract mites: from hay, or from an infected animal.
Many owners state that you should freeze the hay that you buy for 72 hours, then leave it to defrost for a few days before serving it to your rabbits. This period of freezing will kill any mites and their eggs - just make sure you defrost the hay properly afterwards, and never give your rabbits cold, frozen, damp or mouldy bedding.
Mites are bad news for rabbits and need eradicating asap
As well as from hay, mites can also be transferred by other pets in the household. If your rabbit has recently been in contact with another person's rabbit, it is possible that they contracted it from them. Despite their tiny size, mites are very hard creatures and can survive on things that other rabbits have touched, such as toys or bedding. Washing things above sixty degrees, or freezing for 72 hours, will kill off the mites and their eggs (freezing, however, is more efficient at getting rid of mites).