Post 1 

I think there have been some excellent explanations regarding what Giardia is. One thing I thought I should point out (from a Vet's standpoint)....As with any other types of intestinal parasite, Giardia may not always be present on fecal observation. In fact out of the usual intestinal parasites

(Hooks, Whips, Rounds Tapes) the Giardia cysts are VERY tiny even under the microscope and sometimes very hard to pick up on. This makes them one of the harder things to find, even if they ARE there to be found :)

Sometimes more than one stool sample can be beneficial. 

Giardia is a nasty little bug (a genus of protozoa possessing flagella-Taber's Medical Dictionary) that inhabits the small intestine of man and other animals.  They attach themselves to the cells of the intestinal mucosa, from which they absorb nourishment (Taber's Med Dict.)  They are found in water supplies and are not killed by the concentration of Chlorine that is used  in treating these water supplies.  Giardia can be found in mountain streams and this is why you do not drink water from a stream without first filtering it, treating it, or boiling it.  You can become very sick if infected with it and the symptoms are not very pleasant nor is the diagnostic workup.  I don't know about an innoculation for it.
Laurel LeGras with Delilah and Beau