epidemiology

Yes but NOT traveling back in time to cause the black plague results in all kinds of worse timeline shenanigans.
- The temporal trolley problem. Lol
- There's a few of these
  I think the biggest one for me is quite how much support Hitler apparently had in the Anglosphere before the wars.
  Assuming he managed to grow that support without war, I have a feeling we would have potentially had a very fascist 50s across the world instead of the era of progressive politics we actually got. Then whatever inevitable revolution or nuclear winter that followed a couple of decades later, would leave the world looking very different to today.
  
  I often wonder how different the world would be if Hitler had used his power for good.

Travelling forward in time could also kill everyone... Our adaptive immune systems are developed somatically and purifying selection is nonzero in humans.
- Right I think if some bloke brought me some OG plague from the 1200s, my body would have no clue.
  
  Yeah but equally that original plague bacterium has no clue about antibiotics and no resistances. So you'll be fine. As probably will be people in the Future. Or they died out without you anyway.

Oh shit, is that why nobody attended the 2009 Time Travelers party? No one wanted to be the person who killed the great Steven Hawking
- They knew waaaay more than the rest of us did at the time, you don't wanna get MeToo'd for a party you went to 100 years ago, do you?
  
  The guy who speaks with a computer that reads his cheek movements is probably a good candidate for falling under "celebrities they flew in to provide a smoke screen."
  I feel safe in assuming Hawking wasn't actually tied into an oligarchal pedophile conspiracy.

A similar argument could be made for making first contact with an alien species. There's a decent chance one or both species carries some form of microscopic life that the other has no defenses against, assuming both species can even survive in the environment necessary for the other.
Not only will starship captains and crew not be able to have sex with the hot aliens they meet, "they have an nitrogen/oxygen atmosphere" won't mean the away team can land without environmental isolation suits. Planets with oxygen in their atmospheres might be the most dangerous ones out there for us.
- I personally trust humanity that we will work out a way to have sex with something we shouldn't rather quickly. Our horniest scientiest and our brightest perverts will find a way, even if it's at great personal cost.
  
  Obligatory "that Venn diagram is basically a circle."
  
  Yeah, it is probably going to be one of the top 5 reasons to join a starship crew, so life will probably, ah, find a way. For all we know, NASA might even have a focus group working on this already.
  BUT that doesn't mean it won't lead to the extinction of one or both species involved.
  
  Read "scientities"
- I think chirality is something most people overlook in those situations too. Even if you found a world of beings exactly like you, a perfect earth with plants and low CO2 concentrations, if their proteins have opposite chirality to yours, you’re probably going to die of prion disease.
  “Oh look a perfectly human person on an earth like planet I’m sure I can take my helmet off”. Nope. You just inhaled spores or skin cells or pollen or viruses or literally anything that contains “misfolded proteins” and if any of those get at all digested they could cause your body to produce more misfolded proteins, a cycle that will eventually lead to your demise.
  “Look this plant isn’t poisonous” chirality is harder to check than chemical makeup. So yeah it has vitamin B but is it the kind that could kill you? (We don’t have to worry about this much on earth because basically all life on this planet makes and uses proteins of similar chirality)
  “Wow that alien sex was great” too bad there were skin cells in saliva you both exchanged/ingested (or proteins in other bodily fluids) so you’re both going to die now.
  Worst part is that prions are really slow acting. You could all be chilling in this wonderful earth like home for months until around the same time you all suddenly get sick and die. There’s no cure, so there’s nothing you can do besides leave a warning for the next crew who might stop by.
  Oh and the same dangers go for native life on the planet too. To them you’re made of misfolded proteins so any scavengers who eat you and maybe even predators who eat them have a chance of developing and spreading prion disease. Your bodies are basically bioweapons. Any earth crops or animals you brought with would be biohazards too.
  
  Prion diseases are not as simple as coming into contact with a protein composed of D-amino acids.
  While it's still not fully known, it's generally one specific type of protein PrP^{C that gets transformed by PrP}Sc, converting the C to Sc, and so on.
  D-amino acids also occur naturally on earth.
  As for B vitamins, none include proteins as far as I remember, they're all small(ish) molecules like biotin and folate. Also having a difference in stereochemistry with most of these vitamins, wouldn't cause toxicity.
  Obviously there are some cases of small stereochemical changes being toxic (thalidomide). But generally small changes won't automatically make something toxic.
  
  Prions wouldn't work like that. They would have to be very similar, and the same chirality, as our own proteins, or else the misfold would not self-replicate like prion diseases do.
- There's a decent chance one or both species carries some form of microscopic life that the other has no defenses against
  I'd argue there's essentially zero change we'd be biologically similar enough for any microbes to bridge the gap. It's a big deal when microbes jump species here on earth
  
  That's the result of a balance between different microbes and their own defensive measures. Entirely novel microbes and biological functions could overwhelm all of that. Hell, if they have just evolved a next level ATP, they could have access to an order of magnitude more energy than our microbes and bodies can use, in which case we'd probably have no chance unless it doesn't recognize us as food.
  It's not guaranteed to happen, but with 0 data about alternate evolution trees, any reasoning about the odds is speculation (including my use of "decent chance").
  Edit: This would be the case for viruses. Unless they work very differently or our biochemistry has some convergent evolutions, alien viruses should be harmless to us and vice versa. Just like the hundreds of viruses we are exposed to with every breath, most don't react to our cells any differently from how they'd react to wet rocks.
- Not only will starship captains and crew not be able to have sex with the hot aliens they meet
  Not so fast, we do have some convenient barrier device to avoid contamination in this context.
  
  I guess it's possible to have safe alien sex with a full body latex suit and respirator. Though maybe think twice if the alien has sharp spike features.
- Can you explain more about the last sentence how planets with oxygen might be the most dangerous please? I'm interested
  
  Oxygen is reactive enough that free O2 doesn't tend to stay that way. Even to us, too much oxygen is toxic because it starts reacting with things we don't want it to inside our bodies (which is why antioxidants are beneficial). The presence of oxygen in an atmosphere implies there's life producing it (or maybe some other process we aren't yet aware of). It can also imply that there's also life using it to keep it in a balance that doesn't freeze the planet or allow it to destroy any life because oxygen producers keep pumping more of it into the atmosphere.
  And the danger part comes from a combination of alien life maybe having microbes our immune systems can't defend against and the possibly of picking one of those up and bringing it back to other human settlements before the carriers realize they have a fatal infection.
  That second part is what makes it more dangerous than a ship accidentally flying into a star or trying to land on a gas giant, which would probably be fatal to all aboard the ship but not dangerous to humanity as a whole. Even just entering an oxygen atmosphere with a ship that never lands could be bad if that ship must later return home.

Now that I think of it, if you can teleport or time travel and only you go but not the things on you (as is the case with pickier stories)... losing your clothes would be the least of your worries if the countless organisms didn't also go with you on the trip.
Goodbye, gut biome!
- Sounds like philosophy of identity. What is you? Is only things with your DNA you? Will a bunch of dead hair, skin cells, and nails teleport with you? Are the things inside your body you? Is only your brain you and it will get transplanted into a new body in the future?
  
  what about water in your cells? checking out the pyramids being built and showing up as a mummy? maybe tying time travel to biology is a bad idea
  
  You time travel and are suddenly both very hungry and a little more bald, haha.
  
  The Ship of Timeus
- The time traveler's diarrhea

No, due to Earth constantly moving you'll end up in space
-Physics
- You're just knowledgeable enough to know that Earth moves, but not intelligent enough to know that there's no absolute reference frame it moves in respect to.
  If you don't continue travelling with the Earth along its path when you time travel, you could literally end up at any random point in the universe, unless you pick a different, arbitrary, body to move in reference to.
  
  But the universe is also constantly expanding. So the frame of reference becomes obsolete because it's at an entirely different point in space now.
  
  Throw thousands of satellites back in time but each offset. Measure when you get a broadcast from them and how far back you sent them and bam, we find out for reals.
  
  Then there's you who forgot that we actually do have a universal reference frame with the cosmic microwave background.
  
  Nah, I know the thing with reference points, but that's a matter of navigation and relativity.
  In reality, a point in space is a point in space, like, a specific "pixel" of the Universe (oversimplified) that might be occupied with something or not.
  We just can't anchor this point since we don't know what reference is absolute and the laws of physics can be applied to every inertial reference, so this doesn't help.
- Lol, like the people time traveling wouldn't be able to shift the absolute position in space....
  
  hey I invented time travel, but there's one thing I can't figure out... how do you travel in space?
- People say this but the reality is that you are already traveling through time and you aren't teleporting into the space where the Earth used to be. Just as traveling backward would essentially just reverse that process. i.e unless you specifically design a time travel device to kill you that way, thats not how time travel works.
  
  I think a time travel device would, itself, need to be a spaceship anyway. And since it would have to travel FTL to achieve time travel in the first place, getting back to where Earth is at that point in time shouldn't be difficult.
  
  If your time machine works this way, if you're not isolated from reality while you travel, you'll have to do everything at crazy speeds and potentially backwards and you'll be limited by the age at which you're reasonable enough to be able to travel back. You won't be able to travel to times when you don't exist as a person.
- That's why you get a T.A.R.D.I.S.

To be less hypothetical and more scary: Think about all the ancient pathogens that are dormant in the permafrost. Those could become a real problem when it starts to thaw because of global warming.
- There was already an outbreak of thousand year old Anthrax from thawed animal corpses in Siberia several years ago.
- Or any smallpox samples sitting in the back of an old lab, like the ones they found in 2014. Or the smallpox samples that the US and Russian governments keep ~~as WMDs~~ for research purposes.
- The TV show Fortitude has a similar plotline. Would recommend!

This is (kinda) part of The Witcher books. Ciri hops universes and gets stuck on Earth during the black plague. When she hops back to her own universe, she unknownly takes a bit back with her, thus starting the Catriona plague on The Continent.
- Not the actual Earth, right? Just a similar planet.
  
  Nope, actual Earth.

This is a plot point in The Accidental Time Machine: https://www.goodreads.com/book/show/21608.The_Accidental_Time_Machine; the main character ::: spoiler spoiler jumps forward into the future a bunch of times, longer jumps each time, and hits a time where the human population is almost gone, and they're like "yeah, the fucking time travelers keep showing up and bringing old diseases":::
- Also The Witcher

Warp drive will really fuck up a lot of civilizations.
- This is in part why we have the Prime Directive.
  
  That thing that gets routinely ignored by every starship captain ever?

Glove and Boots did a fantastic take on this very concept. https://youtu.be/75nBenOWul0?si=4lTY2at2aHtaqN-F&t=144
- Miss those guys...

Funny but diseases usually become less virulent over time. A successful disease generally doesn't harm the host too much.
Ebola doesn't spread far because it quickly kills the carrier. The COVID-19 pandemic was basically ended because it mutated into a less dangerous variant.

I’ve felt this would be similar for generation ships. If our civilizations get met up again, everyone would probably die.

That must be the origin of Covid, it explains why they can't find where it actually came from.

We're all traveling forwards through time.

This is addressed in The Rise And Fall of DODO. There’s a whole decontamination quarantine period for time travelers.

Schrodinger's epidemic

The bacterias would just reproduce as any other organism

Butterfly effect is a fever dream. A world-wide epidemic and you returning back in time to a different planet is a very very likely scenario.