"Unicellular organisms are living things that exist only as a single cell. This is common in bacteria, some protists. and microscopic fungi.
Unicellular organisms can also aggregate forms or groups called colonies. These colonies are usually observed in bacteria which has a very fast growth rate given a good living condition.
There are also unicellular eukaryotes.Protists like Entamoeba histolytica is one known protist that is also the cause of amoebiasis. That protist is a unicellular organism. As for fungi, yeasts are considered unicellular fungus. They tend to form aggregates, but each individual cell can live on its own diven a good living condition.
For more information about unicellular organisms, amoeba, and protists, you may click the links below: