Sometimes the deadliest part of a viral infection is not the virus itself, but the immune reaction to the virus. So what happens if an apocalyptic mad-scientist type unleashes a viral pandemic that will cause every deathworlder species he could get his hands on to die because their own immune systems will basically liquefy their lungs, gills, or other oxygen-extracting apparatus, but forgets to take into account that a virus that uses strong immune systems to attack will completely bypass higher-level havenworlders?
For the closest thing to a non-mad-scientist real-world example: https://www.theguardian.com/science/2007/jan/18/birdflu.medicineandhealth -- Anon Guest
[AN: There are so many parallels between the 1918 flu pandemic and the current viral apocalypse going on. Up to and including people believing it wasn't that bad, snake oil salespeople, and other people refusing to wear their face masks]
When Humans say their immune systems have a "kill or cure" approach, they aren't kidding. There is something about the most deadly of their diseases that causes the Human body to act against itself. Deadly fevers killing the brain are only one. Swelling in the cerebral cells, destruction of important organ cells, and a little something called a cytokine storm, filling the lungs with defensive fluid.
Disease leaves scars, even when its seemingly harmless. Some lead to permanent disabilities. Many such are invisible to the untrained eye. Such can lay waste to isolated communities.
Welcome to Greater Deregulation Lower South East, where profit is more important than anyone's life. Here, a plague has been raging for decades. They're only now asking for help because they're running out of children to put in the factories. Children who were only in the factories because there were not enough adults to go around.