I had no idea! I thought the whole point of them was that you didn't have to wash them.
This is copied from my employers website:
"Use cloth nappies when you put a nappy on your child. Britain trashes Millions of disposable nappies a year. Nappies take up 1% of Britain?s landfills and they take 500 years to decompose. Cotton nappies can be reused 100 times and decompose in 1 to 6 months. These disposable nappies consume 1,265,000 metric tons of wood pulp and 75,000 metric tons of plastic. After using disposable nappies we are supposed to wash them out, only 5% of us do. Meaning that millions of tons of dirty, possibly disease infected nappies wind up in our so called "sanitary" landfills. In fact three million tons of untreated faeces and urine does not go through our sewage system but into our landfills. Over 100 different diseases are known to be excreted in human faeces, and there is a chance (at least it has not happened yet) that they can seep into our groundwater."