The cysts (protozoan eggs) are ingested orally from a fecal source or route of other contamination such as water. They live in the lining of our small intestine, and the eggs leave the body via feces to be ingested by the next unsuspecting host.
Sometimes the human, dog, etc. will show no signs of this parasite. For others, they may experience cramping, rancid diarrhea, and/or mucousy or frothy stool. The average tech will tell you that the odor of a stool from a Giardia infected patient is very distinct.
Your vet and their techs can diagnose your animal by doing a microscopic evaluation of their stool. Giardia is treatable, and is treated for 1-2 weeks. Your veterinarian will create a treatment regime specific to your dog. Prevention is enhanced by disinfecting your environment (Lysol), and by avoiding drinking untreated water. There is also a vaccine available, but I am not certain as to how prevalent it is.
I think it's key to understand that your dog or cat does not need to contract this directly from eating the poop of an infected animal. An infected dog may have cysts in his mouth from consuming infected stools, and him lapping up water beside your dog may be all it takes for the water to become contaminated.
Giardia is very prevalent, but as mentioned, some dogs will suffer no ill symptoms from it. A research study through Fort Dodge suggested that 10% of all dogs and cats carry it in their systems, 35% of all puppies and kittens, and 80% of all dogs in Colorado carry it. There is no vaccine to prevent Giardia.
It sounds like scary stuff, but remember it's treatable. There are a million reasons to not kiss your dog on the mouth, and always ensure you wash your hands frequently enough to ward off any potential cooties our sweet friends may be carrying. You may notice your vet or tech wearing protective gear over their faces while doing dentals on your pet.. this is why! I admit.. I kiss my dog's face and I am putting myself at risk.. but it's tough. She's just so damn cute. Just remember, a lot can be determined by watching the nature of your animal's fecal output, regardless of how strange it may make you look to your neighbors!