The second most evil creature known to man (see Ticks); anything that has a smiley face on it under a microscope is just creepy. Giardia is a parasitic organism that is common throughout the world, including in developed countries such as Australia and the USA and a strong reason to filter water.
Symptoms include extreme gastrointestinal distress which usually shows up until several days after infection; not the sort of things you want to happen when you’re trying to hike. A medical diagnosis of Giardia in Australia is reportable to the health authorities.
I managed to pick up a really base dose of Giardia in India a number of years ago and never ever want to get it again because it created havoc with my gut for many months after.
Giardia duodenalis, also known as Giardia lamblia or Giardia intestinalis. This parasite is a single-celled parasite