The Kalandans were an advanced space faring species of humanoids. They possessed significantly advanced technology around the 1st millennium BC, such as molecular transporters that could transport large objects light years away, and computers which could create lifelike projections in such distances, that would read minds. Kalandans also built artificial planets for use as outposts.
During the construction of one such planet, the Kalandans accidentally created a deadly organism that infected and killed the outpost's inhabitants. Supply ships servicing the outpost inadvertently transported the disease back to the Kalandan homeworld, which presumably led to the extinction of their species. (TOS: "That Which Survives")
The Star Fleet Medical Reference Manual stated that the Kalandan government was an empire, and referred to the "Kalandan organism" in its section on "Diseases".