Wool diaper covers (also sometimes called "soakers") are made from sheep hair/fibers. When they are enriched with lanolin, the oil that the sheep produces from their skin, wool diaper covers become water resistant. This makes them a great choice for people looking for a natural, more breathable alternative to polyester waterproof diaper covers. While most wool covers should be hand washed, one of the bonuses to using wool covers is that the lanolin used to make them water resistant is also antimicrobial... Meaning that wool covers kind of clean themselves! In general, wool covers only need to be washed if they get poop on them or when they begin to smell of urine when dry (they may smell a little when damp after a diaper change, but that smell should dissipate while they air dry between uses).