A Virus called Prion was considered the smallest microorganism until recently. Though the microorganism hasn't been named yet biologists are pretty positive that it is the smallest microorganism. Follow the related link and take a look at the article.
By definition, a microorganism cannot be seen by the unaided human eye.
dialester pneumosintes is the smallest micro organism & its size is about 0.15 - 0.3 nm
The smallest organism to do so is called a virus
I believe nanobes(of the microorganism family) are the smallest known living organisms at a size of (20 nm = .02 microns) being the smallest.
The microorganism is SAR11, the smallest free living cell known and probably the most abundant organism in the seas
a virus is not a microorganism
The smallest microorganism that causes disease is a prion. It is called an infectious particle. It appears to be a misfolded protein. It contains NO DNA or RNA. It causes "mad cow disease" and Creutz-Jakob disease plus scrapie in sheep and kuru in humans. None are curable and are always fatal. The smallest cellular microorganism that causes disease is a rickettsia. They are similar in size to the mitochondria of eukaryotic cells and both have very similar processes for ATP production. Rickettsia are probably the closest relative (in a phylogenetic sense) to the mitochondria.
fungi is a microorganism.
a microorganism that has decreased in virulence