One point here, not made in “War of the Worlds,” is that the germ exchange will tend to work both ways. The Spanish gave the Native Americans smallpox, the Native Americans gave the Spanish syphilis.
I take it as proof that “alien abductions” don’t take place; otherwise mankind’s population would have been significantly reduced by a wave of diseases to which we had no immunity.
We jusy don’t know if that has happened in past epochs, when earth may have been visited
astro-archeology is turning up more and more evidence of very advanced ancient civilizations that simply vanished
You’re assuming that a race with a vast technological advantage over us would be unaware of viruses and helpless to prevent cross contamination.
BTW I doubt we’ve been visited by anyone.