The Example: War of The Worlds

One thing that humans have going for them is the immense army of malevolent microbial assassins that share their planet. This insidious microscopic army includes ebola, the plague, and the common cold, all of which cause the humans significant amounts of trouble – or did, until the filthy creatures started washing and developed penicillin and a half-decent medical system, damn them.

Sadly, the microbes also keep busy killing off any new species that tries to ‘visit’ the humans. One such species, from the neighbouring red world of Mars, thought they'd be able to bypass the microbe party. They almost did, quickly prevailing in a conflict thanks to their advanced war machines and using humans as fertilizer (HA!). But eventually they succumbed to Terran bugs (with some encouragement from human projectile weaponry), drained and coughing up blood. So very sad, and so close.