Giardia in Humans. Giardia is a microscopic parasite responsible for the diarrhea-related illness giardiasis. Also known as Giardia intestinalis or Giardia lamblia, this organism begins to multiply within the intestines after a person becomes infected.For more details on symptoms of giardiasis, treatment options, and different ways in which humans can become infected with this parasite, click Giardia.).