So, some extinct species. Say, Neandertals. Or mammoths, whatever. We do the Jurassic Park thing, revive them.
So, Matt Ridley, Red Queen stuff.
Don’t they just get wiped out again, near immediately? For the bacteria, virii, bugs and parasites have been evolving these millennia. And the immune systems of those revived have, by definition, not.
Also, time travel won’t work because anyone who travels into the future will drown in their own snot as the then modern cold gets a grip….
That is, our immune systems are the products of a time and place and won’t work well elsewhere or elsewhen.