This one-celled microbe is a real pest. Giardia can get into your guts if you drink contaminated water and cause a disease called giardiasis. The major symptom is diarrhea. Ugh. Giardia have four pairs of flagella (the little pink whips), and they use these to move around. Although Leeuwenhoek discovered Giardia under his microscope in the 1600s, it wasn’t until about 1910 that scientists realized Giardia caused disease. Today, giardiasis can be treated with antibiotics. (Image via Dr. Stan Erlandsen) Read More