Macrocosm (Star Trek: Voyager)

In the Star Trek: Voyager episode "Macrocosm" (Season 3, Episode 12), the Voyager crew encounters a dangerous and rapidly evolving viral entity within a vast, interconnected ventilation system. This entity, which the Doctor refers to as a macro-organism, has infested a previously abandoned Federation starship research facility located within a nebula.

The "macrocosm" refers to the ship and its interconnected ventilation system as a single, albeit artificially created, environment where the viral entity thrives. The ventilation ducts act as a circulatory system, allowing the organism to spread and grow, eventually consuming and adapting to the ship's resources. The Doctor theorizes that the organism is capable of evolution on a scale rarely witnessed, rapidly adapting to countermeasures employed by the crew.

Captain Janeway and Ensign Kim, who are initially unaware of the nature of the threat, are forced to navigate the infested ship, encountering multiple manifestations of the virus, which takes on various forms, including bat-like creatures and aggressive protoplasmic masses. Their objective is to reach the ventilation control system and neutralize the organism before it can spread beyond the derelict ship and potentially threaten Voyager itself. The episode focuses on their struggle to survive and understand the biology of this uniquely dangerous and rapidly evolving threat within its artificial "macrocosm."