• Brainly User
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. 

1 1 1
         they are so small that the cannot be seen using a simple microscope,it should be seen in an electron microscope.they are very hard to classify as a living thing or a non living thing,on their own,they show no sign of life.however they reproduce in the cells of organisms like plants,animals,and,bacteria.tobacco mosaic virus (tmv) and humun immunodeficiancy virus(HIV).

            they are the smallest and oldest organisms on our planet and are found in our planet and are found in four different shape
(i) rod shaped-(bacilli)
(ii) spherical-(cocci)
(iii) curved-(commas)
(iv spiral-(spirilla) 

              they are group of animal-like,single cell organism.amoeba and paramoceium are the examples
1 4 1