How likely is the ending of War of the Worlds?
So I'm watching Steven Spielberg's remake of The War of the Worlds, and it's a pretty good movie with great special effects up to the point where the creepy guy with the shotgun invites Tom Cruise and his daughter to take shelter in the basement. [Warning: spoiler follows. --C.A.] After that you know exactly what's going to happen (icky confrontation with creepy guy, aliens succumb to deadly earth diseases). While waiting for this to play out, I got to thinking: How likely is it that invaders from space would be vulnerable to terrestrial microbes? I know Native Americans were wiped out by smallpox and so on, but they were the same species as European colonists. Animals get lots of diseases we don't get or get only in mild form (distemper, hoof-and-mouth disease). OK, the movie's fiction, but is there any reason to think earthly bacteria and viruses would be able to get their hooks into a species from another planet?
From a technical standpoint let's say it's not out of the question. As a practical matter, however, no. I feel it's important that the populace be made aware of this, or at least the subset thereof capable of grasping the truly big picture: If you're looking to save humankind's butt in the coming intergalactic conflict, don't put your faith in the germs.
First let's deal with what I take to be the crux of your objection, namely that earthly and alien life-forms would likely be so radically different that germs preying on one would be impotent against the other. Here we need to distinguish between the two most common types of pathogen, bacteria and viruses. As far as viruses are concerned, your suspicions are dead-on--the chances of an earth virus successfully infecting an invader from another planet are close to nil. Viruses are essentially bits of genetic code that trick host cells into reproducing them. To do so they need to precisely mirror certain coding sequences in the host's DNA. Even assuming basic biochemical commonality between virus and host--no sure thing when aliens are involved--viruses tend to be species-specific. They do make the leap from one species to another, but only after long coexistence between the hosts. The diseases that did the most to kill off Native Americans--smallpox and measles come to mind--were largely viral. Even now a viral epidemic can be frighteningly difficult to control (witness AIDS) and seems a logical candidate for annihilating an alien horde. Sorry, not gonna happen.
Bacteria are more promising, weaponswise. It's fair to say bacteria, or at least some bacteria, can survive anywhere there's a supply of nutrients, however forbidding the environment. (One classic example: the critters who thrive on hydrogen sulfide emitted by volcanic ocean vents thousands of feet below the surface.) Most bacteria are benign; at any given time your body is hosting hundreds of different kinds without ill effect. Still, some can be lethal, and for purposes of writing a best-selling novel I'd be willing to concede they could take out a creature from the stars.
But here's the thing. Wells's work reflects the leading scientific thinking of its day. WOTW is said to have originated in a discussion Wells had with his brother about the fate of the aboriginal Tasmanians, who were devastated by European diseases. Germ theory had been confirmed only a couple decades before Wells wrote his book; Europeans were just beginning to grasp that the catastrophe they had visited on the rest of the world was less a function of their guns than of their microorganisms. Given that context, the fictional notion of aliens brought low by microbes made sense.
By modern lights, though, WOTW is scientifically naive. Having dabbled in extraterrestrial exploration, we're aware of the danger of contamination and disease (remember NASA quarantining those retrieved lunar rocks?); it's fair to assume invaders savvy enough for space travel would be too. The only way Spielberg's movie makes sense is if the aliens are complete morons, not only breathing our air (they don't wear pressure suits or breathing apparatus) but apparently drinking our blood without benefit of pasteurization.
Get off it, some will say. Kill the germ angle and you kill the movie. To which I reply in three parts as follows: (1) Never mind the movie, birdbrains, we're trying to save humanity here. (2) Even if we stick to cinematic considerations, losing the scientifically dodgy microbes-to-the-rescue gambit would undoubtedly have made the movie's ending less lame than it is. (3) Remember that scene with the fiery train roaring past the crossing, surely one of the most riveting movie moments in recent memory? If Spielberg and his henchmen could dream that up, surely they could have invented a more effective way to save the human race.