Their origin is never explicitly explained. They arrived on Earth as tennis-ball-sized spores drifting down from the sky. Some theories suggest they are aliens, while others (like the character Reiko Tamura) propose they might be a natural defense mechanism created by Earth itself to curb the human population.

When the parasite burrowed into Shinichi's arm, he panicked and tied a cord (headphones/tourniquet) tightly around his upper arm. This prevented the parasite from traveling through his bloodstream to his brain before it matured, forcing it to mature in his right hand instead.

Initially, no. They are driven purely by logic and the instinct to survive and feed. However, the series explores their potential for evolution. Characters like Migi and Reiko Tamura begin to develop complex traits resembling curiosity, maternal instinct, and even a form of empathy or altruism.

While it refers to the alien organisms, the story flips the script. In a famous monologue, a parasite questions whether humans are the true 'parasites' of the Earth, consuming its resources and destroying the environment. The title challenges the viewer to define what it means to be a 'parasite' versus a 'symbiote'.

Without spoiling too much, the parasites realized that open warfare with humanity was inefficient for survival. They adapted by blending in perfectly with human society, ceasing mass slaughters and living quietly to avoid detection and extermination.