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Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length
A virus is a small infectious agent that replicates only inside the living cells of other organism
Protozoa are a diverse group of mostly mobile unicellular eukaryotic organisms, but the term is no longer used in modern taxonomy. 
a mould is a fungus  that grows in the form ofmulticellular filaments called hyphae.In contrast, fungi that can adopt a single celled growth habit are called yeasts.