What is the name of the microorganisms which reproduce only inside the living cells of other organisms?

Viruses are the microorganisms that can reproduce only inside the living cells of host organisms. Unlike bacteria, fungi, or protozoa, viruses are not technically considered fully "alive" because they lack cellular structure, cannot carry out metabolic processes independently, and possess no machinery for self-replication. A virus consists simply of genetic material (DNA or RNA) enclosed in a protein coat, and sometimes an outer lipid envelope.

Once a virus invades a host cell, it hijacks the cell's metabolic machinery to produce multiple copies of itself. The viral genetic material takes control of the host cell's ribosomes, enzymes, and energy resources to manufacture new viral particles, which then burst out to infect neighboring cells. This obligate intracellular parasitism explains why viruses cause diseases ranging from common colds to serious conditions like influenza, AIDS, hepatitis, and COVID-19. Because viruses depend entirely on host cells for reproduction, they cannot survive or multiply outside living organisms for extended periods.