The macrovirus was a viral organism encountered by the crew of the Federation starship Voyager in the TV series Star Trek: Voyager. This organism was initially microscopic like any other virus, but at some point had evolved to adapt the growth hormones of the creatures it infected, allowing to grow to macroscopic size. The macrovirus appears in the Voyager episode "Macrocosm".
The macrovirus was first encountered by the USS Voyager in the Delta Quadrant in 2373. The ship received a distress call from a Garan mining facility that was experiencing a viral outbreak. Voyager's Emergency Medical Hologram, incapable of being infected, transported to the surface where he observed that the virus had entered a new phase in its evolution. The organism had adapted to incorporate the growth hormones of its host into its own amino acid structures. This allowed the virus to escape from the microscopic world and into the macroscopic world. The Doctor returned to Voyager to access his laboratory, but accidentally transported one of the organisms with him, thus leading to the crew becoming infected as well.
Upon infection, the host organism developed a fever and soon fell unconscious. As the virus progressed it concentrated in the victim's neck, where it "hatched" through the skin and became airborne. At this stage the virus was approximately .5 mm, but before long it reached nearly a third of a meter, and could eventually reach a meter if not destroyed.
The Doctor was able to develop an antigen for the virus, but was at first unable to distribute it by himself. Fortunately, Captain Janeway and Neelix had been away on a diplomatic mission when the virus originally attacked the ship; although Neelix was infected while he and Janeway were trying to determine what had happened, Janeway escaped infection long enough to make contact with The Doctor. Although their original plan was to release the antigen via the ship's environmental systems, when the Tak Tak attacked Voyager to prevent the spread of the virus, causing damage to the ship's power systems, Janeway had to improvise, creating a holodeck simulation to lure the virus into one place. With the virus contained in one area, Janeway threw an antigen bomb into the holodeck, destroying the virus.
Under a microscope, the macroviral cell appears circular with three tendrils outstretching from it. When it infects a host, it assimilates the host organism's growth hormones, allowing it to grow to a macroscopic size and interact with the world as humans and other such life forms can. The macroviral organisms reached roughly a meter in diameter with a mostly spherical shape, similar to a basketball in size. It could fly through the air and made a distinct buzzing sound as it flew despite its lack of wings. Its tentacles may somehow have allowed it to move through the air but this is unconfirmed.
Macrovirals would attack other organisms and attempt to infect them with a needle-shaped organ, possibly a proboscis. At such a size, it was possible for a macrovirus to kill a potential host with its proboscis by stabbing a victim.
As macrovirals were large enough to affect the world on a macroscopic scale, they were also vulnerable to macroscopic attacks. When the virus infested Voyager, Captain Janeway killed several macrovirals by shooting them with a phaser rifle and even stabbed one with a knife.