Humans are not great because of what most people think. It’s not our physical capabilities or our ingenuity, not even or pack-bonding skills. It’s our heartiness and our healing ability that is boosted though the advancement of medical applications. Humans figured multiple ways how to rewrite our genetics codes before we had proper space travel, we were able to eradicate deadly diseases off the face of the planet. Hell I wouldn’t be surprised if we created the prototype livesuit. But there always one thing that we’re unable to cure. When you hear the news it hit hard. When a disease is perfectly attuned to your body and you tried to cure it, you’re basically asking someone to mangle your body to have a chance to survive. Cancer is humanity's greatest rival. -- Anon Guest
Humans have a reputation for being unstoppable, and part of that is their ability to bounce back from injuries that other species would consider fatal. The other is their resilient immune system, their kill-or-cure biological strategies that sometimes come close to the first option, then the Human in question manages to pull through.
It's not perfect. Nothing ever is. The Human immune system can turn against itself, cannibalising otherwise perfectly functional body parts, sometimes destroying the Human in the process. Sometimes, it creates more material at random, growing bone where there was once muscle. Sometimes it just eats the body a little at a time. The other way it can go is growing new cells with amazing rapidity. Those clusters of cells take resources from the rest of the Human suffering from their growth and, eventually, starve the entire being to death.
Humans have been trying to stop things like this happening since they realised what those things actually were. Early attempts - including divine intervention and rudimentary yet impossible magic - were not effective. Later therapies were hit and miss until their medicine figured out some precision. For the most part, for centuries at a time, Humans relied on a mixture of highly dangerous medical treatments and equally dangerous surgeries to rid themselves of the anomalous cell clusters. When it comes to "kill or cure", Humans really commit.