We assume that microbes evolved to attack humans when actually we are just civilian casualties in a much older war

Ed Yong in Aeon:

The classic novel by H G Wells, The War of the Worlds (1898) – a tale of England besieged by Martian conquerors – ends not with a rousing and heroic victory but an accidental one. The aliens subjugate humanity with heat rays and black smoke but, at the height of their victory, they die. Their machines come to a standstill amid the ruins of a deserted London, and the birds pick at their rotting remains. The cause of their downfall? Bacteria. As the novel’s unnamed narrator says, they were ‘slain, after all man’s devices had failed, by the humblest things’.

Wells’s logic was simple. Humans have immune systems that protect us from the infectious germs that we’ve been exposed to since our earliest origins. We still get diseases, but at least we can put up a fight. The Martians, despite their technological superiority, could not. ‘There are no bacteria in Mars,’ the narrator explains, ‘and directly these invaders arrived, directly they drank and fed, our microscopic allies began to work their overthrow.’

When I first read the book about two decades ago, this final twist seemed like a cop-out. It comes out of nowhere – a sort of deus ex microbia rescue – and, besides, Earth’s microbes could not possibly grow in an alien body. But more recently, I have come to realise that Wells, writing at the close of the 19th century, was inadvertently hinting at a truth about bacteria that even today’s microbiologists sometimes forget: these organisms can become lethal through evolutionary accidents.

More here.