From Creatures Wikia
Brian had a mutation in the gene controlling glycogen -> glucose conversion such that it only worked if he was lonely. As a result, several things went "wrong" - his energy levels (measured by glycogen) went through the roof, while his glucose level was rock-bottom. He was not usually harmed by disease (which depletes glucose and therefore glycogen), however he could not fight it off either, since antibodies are created using glucose. Of course, this meant that he was a great way for disease to spread!