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Micro-organisms are the smallest living organisms, they come from the word "micro" meaning very little. But however, the smallest living organism is not yet discovered. But virus, I think is the smallest micro-organism, the smallest of it is called "Porcine Circovirus" it has only 30nm length. if it divides, it would be 15nm lenght. 
Here's a explanation. In all organisms, micro-organisms are the smallest. In all micro-organisms, virus is the smallest. And in all virus, Porcine Circovirus is the smallest. So it means Porcine Circovirus is the smallest living organism ever! But the smallest organism is not yet discovered. :3