I teach the Duke of Edinburgh Gold Expeditions and for the first time in 30 years we had an outbreak of Crypto on one of the groups while in the Welsh Mountains, which caused the students to be ill for two complete weeks. This could have been because they failed to boil their water properly as instructed (they ran out of gas) and didn't use tablets, but it can also be picked up from the ground. Either way, they were very ill. The severity may also be because they were fairly young (16-18) and hadn't got any sort of immunity. Like many others, I have used Millbank bags and boiling and have never had a problem - this year I spent two months on an expedition in the Amazon jungle and found this means very good - although people often forget that to get the best from a Millbank bag you need to brush it hard whilst it is wet on the outside with a stiff brush before it will give its best flow. We had to purify in excess of 150 ltrs per day for our team and used the largest milbank bags to remove sediment, and then aquatabs to purify it - this could then be used for cooking or drinking cold.