Giardia is a microscopic parasite that can make people and animals sick. It causes an infection called giardiasis, which affects the intestines and can lead to stomach pain, diarrhea, and nausea. Full definition
To avoid causes of giardia in dogs such as feces, practice good hygiene such as bagging and disposing of feces in bags, and removing and disposing deposited in outdoor areas such as the backyard. (dog-health-guide.org)
There is a vaccine available for Giardia in dogs and cats, but most vets don't recommend it unless your dog / cat is at really high risk or one of those pets that gets it frequently. (vetlive.com)
Common signs and symptoms of Giardia infection (in both humans and pets) are diarrhea, gas, abdominal discomfort, nausea, and vomiting. (tworockdogranch.com)