There are other entities in the “twilight zone of microbiology,” including transposable elements, viroids, unculturable (but putatively existing) microbes, organisms in vegetative states, and prions (Postgate 1999). The problems facing each of these are similar: several of them can evolve by natural selection, are biochemically complex, but lack other properties associated with life. For example, prions are protein products of life that can fold other prions in a way that allows for cumulative evolution (Li et al. 2010). They are rarely included in the category ‘life’ due to their inactivity in most settings and rather simple origin as a misfold of a functional protein.