Here's why:
* Obligate: They *must* use a host cell to replicate. They cannot reproduce independently.
* Intracellular: They live and multiply *inside* the host cells.
* Parasite: They benefit from the host, while harming the host in the process.
Examples of obligate intracellular parasites include:
* Viruses: They are the most well-known example. They inject their genetic material into a host cell, hijacking the cell's machinery to make more virus particles.
* Some bacteria: For example, *Chlamydia* and *Rickettsia* species are bacteria that can only reproduce inside host cells.
* Some protozoa: For example, *Plasmodium falciparum*, the parasite that causes malaria, spends part of its life cycle inside red blood cells.
