If our own immediate history of first contact encounters bears out, millions or billions of earthlings will be wiped out by some alien contagion.

And that's if they aren't armored leech-like bloodsuckers latching onto our brains for sustenance - which would be considered a congenial exchange on their world(s).

We'll just have to wait and see what first contact will be like. But we are probably the Aztecs in this exchange.