Answer: Balantidium coli
This organism is the only ciliated parasite to infect humans. At 80 microns or larger in longest dimension, it is also the largest protozoan to infect humans. Shown here is the classic ciliated oval trophozoite with 'sausage' or 'kidney bean' shaped nucleus.
When I first learned the B. coli had a "boring" motility, I concluded, literally for months that that meant "uninteresting"!
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