Research backcountry water treatment and you’re sure to be warned about cryptosporidium or “crypto.” And for good reason. This microscopic protozoan parasite is one of the most common causes of waterborne disease in humans in the United States. Like the parasite giardia, crypto is found in water sources worldwide, and affects individuals differently. Fortunately, the disease it causes is rarely life-threatening in healthy adults. In fact, some 80% of the U.S. population has had cryptosporidiosis at some time, according to the FDA. Still, its symptoms are nasty enough that you’ll want to take strides to avoid it on your next backpacking trip.