Lore[]
It has been while, 
Mundo thought, stroking the massive purple tongue that hung from his mouth like an executed criminal swinging from gallows, since Mundo made a housecall.
He rolled out of his bed (a large wooden box filled with sharpened knives and rusty nails) brushed his teeth (with a nail file) and ate breakfast (a cat) Mundo felt exuberant. He felt alive.
Today was a fine day for practicing medicine.
He spotted his first patient hawking shimmerdrops just outside Ranker's Limb Maintenance. The man limped around in a circle, shouting at everyone within arm's length about how shimmerdrops would make their eyes roll into the backs of their heads and how if they didn't buy some right now, right this second, then they were damn idiots and did you just give him a condescending look? Because he'll kill you and your family and your family's family.
Mundo took out his notepad, a tool he often used to mark down observations about his patients, both past and present. The notepad was large, yellow, and imaginary.
Patient exhibits signs of mania, Mundo would have written if he hadn't been tracing random squiggles in the air with a meaty finger. Possible infection of nervous system via cranial virus, he might have inscribed if he were capable of such multisyllabic thought.
"Mundo cure head and face area good", he said to himself.
Rank was just about to pack up his shimmerdrops and head home for the night. He needed to get new shoes. These ones rubbed his feet raw when he walked, and at the end of a long day's work, hadn't he earned the soft leather of grayeels?
As Rank was thinking this, a huge purple monster jumped out of the shadows and yelled "Mundo has results of your blood work."
Mundo left his first patient more or less as he found him (save for a few limbs) and took to the Commercia Fantastica, a market specializing primarily in gearwork toys. Though most of the shops were closed, Mundo spied a lone Zaunite teetering to and fro as he stumbled down the path. The Zaunite sang a song of a Piltovan beauty and the shy boy from the undercity who loved her, except he seemed to have forgotten most of the words apart from 'big ol' eyes'and 'gave it to her'. An empty bottle dangled from his hand, and he looked as if he hadn't had a bath in months.
Was this man afflicted by the same disease that had ravaged the shimmerdrop dealer? Was this a virus? An epidemic in the making? Mundo had to act fast.
This was clearly a man in need of medical attention."Take two of these and talk to Mundo in morning", the purple monstrosity said as he tossed a
meat cleaver into the drunk's back.
Mundo descended into Zaun's Sump level. If there was a virus going around, chances were it originated here. There must be a patient zero somewhere. If he could just cure the first sufferer of this mystery disease, Mundo knew he could cure the rest of Zaun.
But how was Mundo to find one specific patient in the sprawl of the Sump level? What steps would he take to isolate, quarantine, and fix this most suffering of Zaunites? How would he---
Mundo heard something. Footsteps, and a rhythmic clang of metal against metal.
He followed the noise as carefully and quietly as he could - wouldn't want to spook the patient into running away and infecting even more people - and found exactly what he was looking for.
A young boy. No older than fifteen, probably, with a shock of white hair and a large metal sword-looking-thing in his hand. He had some sort of hourglass tattooed onto his face. Maybe a warning? A symbol that he was not to be approached under any circumstances?
Mundo knew he'd found him. 
Patient zero.
"You might feel a little sting", the creature said, leaping out. His enormous purple form hurtling through the air, massive cleaver in hand, tongue flapping in the wind.
The boy was surprised, but not unprepared. Anybody hanging out in the Sump knew to be ready for trouble at a moment's notice, and the kid had plenty of time to prepare.
Nothing but
time, in fact.
No two ways about it: this was a troublesome patient.
He refused to answer Mundo's questions about his medical history, and repeatedly evaded Mundo's attempts to make him take his medicine. He repeated himself over and over again (perhaps suffering from a case of physical amnesia?) and had no respect for Dr. Mundo's authority.
The two scuffled over the child's sickness for what felt like hours. Mundo made what he thought were very salient points about the merits of treatment, but the child constantly evaded Mundo's attempts to help him.
Mundo grew tired of arguing with the boy. He mustered up one final attempt at treatment, wielding his precision scalpel with the artistry of a Demacian duelist. The words of his medical vows - "Mundo fix all things, Mundo do medicine very hard" - ran through his head again and again. His desire to cure this child filled him with purpose and determination.
He swung with all his might.
The treatment was a success.
But then - somehow - the treatment 
reversed itself. Whatever good Mundo had accomplished in his last attempt at a cure was suddenly undone. To Mundo's utter confusion, the child scurried away, utterly uncured.
Mundo screamed in irritation.
"Why can't Mundo save them all?" he screamed to the sky.
Not every operation was a success. Mundo would be the first to admit that. Still, Mundo tried to focus on the positive. Apart from this most recent patient, Mundo had helped an awful lot of people. He'd done a full day's work, and now it was time to rest.
As the sun came up, Mundo retired home and tucked himself into bed. Who knew what tomorrow might bring? Another patient to help. Another epidemic to stop.
A doctor's work was never done.
Trivia[]
- This story describes Ekko's Z-Drive with the ability to locally reverse damage to his body rather than reverse the timeline to do so, which is not displayed in all other appearances.