Mainly, you have to drink some water harboring these giardia cysts. "Drinking," though, can be loosely defined. It's possible to contract giardia through accidental ingestion of bad water, such as when swimming in a stream. I'd also include toothbrushing in this category—you wouldn't want to walk up to a clean-looking stream and dip your toothbrush in. Instead, use a little treated water.
Ditto for avoiding cryptosporidium, which has a life cycle similar to giardia—passing out of infected animals and humans in feces, infecting water supplies, then starting over again when a human or animal drinks the tainted water. Cryptosporidium cysts are also very tough and can survive even in hot tubs. But they require moisture, and basically all cases of human infection are traced to drinking infected water.
What about incidental contact, such as washing your hands? Some experts advise you not to wash in water that may be infected, or if you do to use a hand sanitizer. I think that's a little excessive, given the fact these parasites almost always require the ingestion of bad water, but you can take it for what it's worth. As for your cookpot, it's not likely to harbor parasites when it's dry, and any that may exist in a damp corner will be killed when you heat your dinner.
In short, be prudent. But there's no reason to be paranoid.
Find out more about the microscopic world of water-borne nasties and tools to combat them in Outside's 2004 Buyer's Guide .
Filed To: Water Treatment