What is an organism?
The term organism has been up for debate for quite some time now. Only living things now will be called organisms. Viruses do not fit the term.
Mycoplasma gallicepticum , a parasitic bacterium (200 to 300 nm) which lives in the primate bladder, waste disposal organs, genital, and respiratory tracts, is thought to be the smallest known organism.
Just for fun information : The smallest virus known to humanity, however, is the single stranded DNA virus Porcine circovirus type 1. It has a genome of only 1759 nucleotides. Its capsid diameter is only 17 nm. It causes wasting disease in newly weaned pigs.