Giardia is really common and is usually spread through fecal contamination. Often you have to treat, wait a bit, test the stool to make sure it is gone... Do not get discouraged if you have to retreat because unlike most parasites, giardia is a little different. Generally the life cycle must be interrupted... hence the retesting. The adults stop reproducing and die off.
Good hygiene is important. It is zoonotic (like almost all parasites) meaning it can be transmitted between species. So wash your hands a lot and if you have any children, make sure they wash their hands often too. Occasionally the cysts (egg form) will stick to the fur around the rectum. Blah... also make sure and immediately clean up any "piles" in the back yard or wherever Jed will go potty. This will help avoid re-contamination. I know some vets even recommend wiping off their feet after they come in from outside.
It is very important to finish all the medications as directed by the vet and to retest the stool sample even if your dog isn't symptomatic because they can transmit giardia even if the stool is normal. Giardia can be tricky... but it is definitely curable.