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, 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; Neelix was infected while he and Janeway were trying to determine what had happened, but Janeway escaped infection long enough to make contact with The Doctor, giving him a suitable test for his antigen. 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 by providing it with a new source of apparent hosts. With the virus contained in one area, Janeway threw an antigen bomb into the holodeck, destroying the virus. (VOY: "Macrocosm")
When Voyager was split into multiple timelines by a temporal anomaly, the Chakotay of 2377 and the Janeway of 2371 encountered a macrovirus in the corridor in one of the timeframes, although they were able to evade the virus long enough to retreat to another timeframe where the virus couldn't reach them. (VOY: "Shattered")