Deicitis

Deicitis is widely known as the galaxy's worst bacteria which is an alien-class specie which is incredibly rare as bacteria are usually found in the birth-class which has wiped out countless civilisations including alien-class species in a large number of solar systems and are still roaming the galaxy in search for victims however, they tend to avoid species among the alpha-class along with species in higher-up classes so, are unlikely to target Earth as members of the IG's high council are said to be here to research the native species.

Origins
As shown by the Nature Theory, ascending to the alien class naturally is virtually impossible and must have involved artificial modifications either from within the specie or from another specie although, scientists have calculated a high probability due to the exponential speed in evolution that another specie originally modified the Deicitis, probably from a harmless bacteria to this threatening level of development as such incredible development speeds are extremely rare without external intervention from another specie.

Spread
We've also, calculated that there's a high probability that the Deicitis may have spread to galaxies outside the Milky Way as this bacteria not only wants to spread to gather as many victims as it possibly can but, it also has the means to do it having evolved to the alien class and so, it would have no problem or even that much difficulty in spreading into the intergalactic space as they tend to avoid the powerful aliens which makes it harder to us to track them but, not entirely impossible.

The IG however, is not that interested in tracking and destroying Deicitis though as that would be technically interfering in the affairs of the aliens with the aliens having the right to invade other aliens.