Given that our viruses might not attack aliens with differently coded DNA and RNA, and that many of the diseases that attack human beings are transmitted by insects, ticks and lice that suck our blood, and might not find E.T.’s blood appetizing or even edible, do visiting or invading extraterrestrials get a free pass? Hardly. There are, along with the more specialized parasites that infest planet Earth, a number of microbes, particularly bacteria, that infect a wide variety of organisms, both plant and animal, and that are extremely opportunistic. Three genera of opportunistic bacteria that might plague E.T. come readily to mind: Staphylococcus, Streptococcus and Pseudomonas. All of these have in common a number of different species that attack a number of different types of organisms, increasing resistance to antibiotics and the ability to subsist on a wide variety of compounds in a number of different environments.

Consider Staphylococcus. Necrotizing fasciitis, commonly referred to as flesh-eating bacteria syndrome, is often caused by methicillin-resistant Staphylococcus aureus (MRSA). While this disease is comparatively rare, it highlights the marked resistance of S. aureus and other Staphylococcus species to antibiotics, as well as the opportunistic nature of this form of bacteria. Various species of Staphylococcus infect birds and mammals. It would seem likely that extraterrestrial endothermic (warm blooded) species could also act as their hosts.