The Whaela'an was an intelligent virus accidentally enlarged by a Diagnosan named Cotat. The Whaela'an began engaging in subtle, biological warfare on numerous worlds in an effort to conquer much of space -- manipulating and infecting translator microbes and other common, single-celled organisms. This would cause hysteria and madness on the worlds the Whaela'an were invading.
Ultimately, the Whaela'an were stopped when Cotat created an anti-virus, tested it on John Chrichton, and had it spread by the Hynerian Empire.
The Whaela'an is derived from the Farscape comic books.