Giardia and Giardiasis Giardia and Giardiasis: Understanding the Basics Giardiasis is one of the most common intestinal parasitic infections worldwide. While it is often associated with contaminated water and outdoor exposure, it can occur in everyday settings as well. Understanding how Giardia spreads, who is at risk, and what symptoms to watch for can help with early recognition and proper care. What Is Giardia? Giardia intestinalis  (also known as Giardia lamblia  or Giard