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 alos have origionally been such parasites, but ended up forming a symbiotic relationship.

Parasitology