Viruses that grew to visible size and flew through the air, infecting the crew of Voyager and forcing Janeway to fight them almost alone.
A macrovirus begins like an ordinary microscopic virus, then enlarges using its host's cells until it becomes large enough to see, fly and attack people directly. Its victims fall seriously ill.
In Macrocosm, Janeway returned from a mission with Neelix to find Voyager overrun and most of the crew unconscious. Working with the Doctor, who had developed a treatment, she battled the creatures through the ship and destroyed them by flooding Voyager with an antigen.