Giardia and Coccidia are single cell organisms. They find their way to the dogs intestine and that is when the trouble starts.
With both dogs must ingest a the cyst from the parasite. But this does happen and before you know it the dog is infected.
Coccidia is seen more in puppies then adult dogs. Puppies are not born with Coccidia, but they can become infected from their mother.
Many dogs that come in to shelters as strays, or those who live outside in unclean conditions, with unclean water to drink come in bring Giardia with them.
-Lack of appetite