Ballboids are alien parasites that envelop and slowly digest their host life-forms, a process that takes several months. When the ballboids encounter a suitable planet, they start with most the advanced life-form and work their way down, eventually consuming all life.
Original Series Edit
In "The Living Skins", the trendy new fabric bubble-skin was a deception by the ballboids to take over humanity. John discovered that the ballboids were not immune to the common cold, and used the cold virus to drive the aliens back into space.