This challenge was different -- Chuck Wendig is doing a worldbuilding exercise. The entries are all in the comments, but I thought for completism, I'd post mine here, too.
The world was once terraformed. Aliens had seeded it With algae spores. These spores grew on all the wet things, killed some of them, and converted others. It was a very painful process. Creatures walked around, bodies half-covered in algae, going mad from pain.
The algae spores are a modified version of filamentous green algae, which does conjugal reproduction (trading DNA with other species). The algae takes the sulfur out of the SO2 atmosphere, leaving the free oxygen that the original lifeforms are allergic to.
The algae is still out there. Occasionally there’s an outbreak. Non-natives are particularly vulnerable.