Space ship vermin

Space Ship Vermin are a group of lifeforms that have adapted to living within spacecraft but who's presence is mostly not desired. They can be classified into four groups based on how much of a danger they are to the desired contents of the spacecraft.
 * 1) Harmless/Slightly beneficial - This group is sometimes brought into a spacecraft intentionally, and are only considered vermin due to the non insignificant risk of their numbers getting out of control at which time they start to strain the life support systems.
 * 2) Mostly Harmless.
 * 3) Slightly harmful to Harmful.
 * 4) Extremely harmful.

Examples: Bonglee