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So, let's say you have two planets in parallel universes. Travel is possible between Earth and the second planet via wormholes or whatever (not important). While the oxygen-nitrogen-et cetera ratios and air pressure are similar enough to be survivable by both species, there remains the issue of bacteria. Earth's atmosphere is chock full of all sorts of diseases that we don't even consider diseases due to our natural immunity or their tendency to infect things like plants or dirt or fish instead of humans, but then stuff like the common cold is known to kill gorillas apparently. Of the bacteria in Earth's atmosphere (discounting staph), how many of them are likely to actually be deadly to someone with a human-like immune system that's never encountered them before? Worst-case scenario, someone runs out of their own air supply and has to take off their helmet and breathe the local air to avoid suffocating immediately. How boned are they?

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marked as duplicate by Rob Watts, L.Dutch, Community Mar 4 '18 at 7:10

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  • $\begingroup$ If this new world has an atmosphere "similar enough to be survivable by" humans, then our intrepid explorers will be wearing air filters, not air supplies. $\endgroup$ – RonJohn Mar 4 '18 at 5:20
  • $\begingroup$ As far as survivability, assuming that this exobacteria is also based on DNA and can use humans for food just like Earth bacteria... we'd have just as good of odds of survival as Earth humans do when exposed to new bacteria here on Earth. $\endgroup$ – RonJohn Mar 4 '18 at 5:24
  • $\begingroup$ Thanks! Though I'm not certain that's a good thing -- just look at how the whole Christopher Columbus thing went. $\endgroup$ – Nautilusopus Mar 4 '18 at 5:26
  • $\begingroup$ That's exactly what I had in mind... :) $\endgroup$ – RonJohn Mar 4 '18 at 5:35
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    $\begingroup$ If the two planets are connected by rabbitholes or whatever, then chances are good both planets have already long been infected by each others' microbes as they wander from one to the other. $\endgroup$ – elemtilas Mar 4 '18 at 5:46
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Honestly, unless there's something wildly improbable going on, the alien bacteria aren't even remotely a threat.

Earth-derived bacteria are a threat to humans because bacteria and humans have four billion years of shared ancestry. They use the same genetic encoding, build their proteins out of the same amino acids, use the same sugars for energy, use the same sort of lipid bilayer to enclose their cells, and so on. That shared ancestry means an Earth-derived bacterium sees a human as a massive pile of resources to eat (and why a human needs an immune system to protect itself).

Now, what are the odds that a random planet's biochemistry is similar enough to Earth's that the bacteria there can consume Earth-derived organic materials? Even a biochemistry based around similar classes of molecules (carbohydrates, amino acids, and so on) is almost certainly going to be incompatible, simply because there are so many possible molecules within those classes.

The biggest threat is probably that the alien bacteria will find your lungs make a great surface to grow on, and proliferate to the point that you suffocate. The odds of getting infected by an alien disease are effectively zero.

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  • $\begingroup$ "what are the odds that a random planet's biochemistry is similar enough to Earth's that the bacteria there can consume Earth-derived organic materials?" Quite high, seeing that amino acids have been found in space. newscientist.com/article/… "An amino acid has been found on a comet for the first time, a new analysis of samples from NASA’s Stardust mission reveals." $\endgroup$ – RonJohn Mar 4 '18 at 6:47
  • $\begingroup$ @RonJohn, amino acids are a huge family of compounds, of which Earth-derived life uses 22. An amino acid consists of a central carbon atom bonded to an amine group (hence the name), a carboxyl group, and a side chain; the side chain is what gives the amino acid its distinct properties. Virtually any carbon structure can show up as a side chain, and because there's such a huge variety of such structures, the odds of an alien biochemistry using the same set is virtually non-existent (if it even uses amino acids at all). $\endgroup$ – Mark Mar 4 '18 at 7:12
  • $\begingroup$ "of which Earth-derived life uses 22." ... "the odds of an alien biochemistry using the same set is virtually non-existent" Why does "Earthican" life use the 22 amino acids that it does? From random chance, or because those 22 support life-enabling chemistry that the others don't? $\endgroup$ – RonJohn Mar 4 '18 at 7:54
  • $\begingroup$ At least to some degree, random chance. Earth-based life uses the "L" stereoisomers, but life using the "D" isomers would be just as viable (and totally inedible). $\endgroup$ – Mark Mar 4 '18 at 9:21
  • $\begingroup$ So we're down to a 50% chance of 90% (from the apparent % of aboriginal Americans who died) of humans on that planet dying. $\endgroup$ – RonJohn Mar 4 '18 at 14:05

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