Prof Paul Davies’s headline-grabbing proposal of an invasion by alien viruses (Viruses may exist ‘elsewhere in the universe’, warns scientist, 6 September) may well raise a scientific eyebrow or two.

We can be confident that living cells emerged before viruses (on any planet) because viruses are, by definition, obligate parasites: they can only multiply inside a living cell. Even if you say that viruses were originally cells that then became stripped down, you are merely confirming that cells came first. And whereas the cells of some species of plant and animals have evolved to endure extreme conditions, viruses are, by contrast, rather delicate things. For example, the Sars-CoV-2 virus cannot keep going, after leaving its host, beyond about two metres. Even just two light years of intergalactic space is likely to be a bit of a stretch.
Martin Yuille
Honorary reader, epidemiology and public health group, University of Manchester