Obligate intracellular parasite

Obligate intracellular parasites are parasitic microorganisms that cannot reproduce outside their host cell, forcing the host to assist in the parasite's reproduction.

Obligate intracellular parasites of humans include:
 * Viruses
 * Certain bacteria, including:
 * Chlamydias
 * Rickettsias
 * Certain protozoa, including:
 * Plasmodia species

The mitochondria in human cells may also have originally been such parasites, but ended up forming a symbiotic relationship.