The microscopic macrovirus.
The macroscopic macrovirus.
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. The Doctor, 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. The crew became 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 and after initial difficulties distributing it to the infected crewmembers of Voyager was able to cure the crew with the help of Captain Janeway.
The Tak Tak were also aware of the virus, however, they had no cure for it. As a result they were prepared to destroy Voyager to prevent its spread. (VOY: "Macrocosm")
Chakotay and Kathryn Janeway also encountered a macrovirus in the corridor in one of the timeframes that Voyager was split into. (VOY: "Shattered")