What term is used for ultramicroscopic parasites composed of proteins that can multiply only in living tissues?

The term referring to ultramicroscopic parasites that are composed of proteins and can replicate only within living tissues is indeed a virus. Viruses are unique in that they lack the cellular structure found in other types of microorganisms, such as bacteria, fungi, and protozoa. Instead, they consist of genetic material encased in a protein coat, making them much smaller than these other organisms.

Viruses are dependent on host cells for reproduction, as they cannot carry out metabolic processes on their own. When a virus enters a living cell, it hijacks the cell's machinery to produce copies of itself, which then can lead to cell damage or death and can spread to other cells.

In contrast, fungi are mostly multicellular organisms that can thrive in a variety of environments, bacteria are unicellular organisms capable of independent life, and protozoa are single-celled eukaryotic organisms that can perform metabolic activities independently. Each of these types of microorganisms has distinct characteristics and life processes that differentiate them from viruses.