The plague

 This week it's been announced that scientists have brought back to life a 30,000 year old virus thawed out from the Arctic permafrost.  Their goal was to see what could happen if the Arctic ice melts and all the other dormant viruses there were returned to life.

We only think of viruses when we have a bad cold, but delving back into the role of viruses in the history of the Earth and humans brings some interesting insights. We already have a problem combating ever-mutating 'flu strains.  It's not too big a stretch to imagine a newly-awaked Arctic virus that infects the birds that migrate there.  When those birds go south on their winter migration, they take the virus with them to a completely new set of animals.  Perhaps our domestic herd beasts contract the virus, and then it's passed on to their human herders...

For those of us who believe in the Gaia hypothesis, this kind of mechanism would be an ideal way for the beleaguered Earth to thin out the numbers of the human pest that is overrunning its surface.  Perhaps the next apocalypse will come before we run out of food or water, but be carried by tiny biting insects.

Our gathering together closely in cities makes it much easier for viruses to spread from host to host.  But we could get the opposite effect too.  If the citizens of a large continent evolved an immunity to a major disease, anyone moving in to conquer them from elsewhere would be likely to be attacked by the disease and the invasion would probably fail.

That happened to Napoleon's army in 1802 when it invaded Haiti.  His troops were decimated by yellow fever, a disease that mosquitoes transmit.

Assault team commanders might think twice before sending teams down to alien worlds.  The mightiest army is capable of being killed by the tiniest creature.