source
According to a recent study, many common protozoans living in the human gut are extremely contagious, and likely to be widespread in many animal species, causing unsettling digestive disorders.
The study was conducted by the Norwegian School of Veterinary Science, in early 2009. They found that parasites of the groups Cryptosporidium and Giardia are enormously widespread [...]

source
Of all the components of the human anatomy, by far the most delicate and susceptible to upset is the digestive tract. Giardiasis is one such ailment that affects the digestive tract. The ailment is triggered by the presence and action of the microscopic Giardia Lamblia parasite.
The Giardia Lamblia parasite lodges within the delicate lining of [...]