The pandemic was kicking McCoy's ass. There was one main institute where he was working, which had several thousand dying patients inside of it. He worked as rapidly as he could, administering hypo after hypo. There were about a hundred people from the Enterprise that offered to help, so he wasn't completely alone in the endeavor, but it was still difficult. All these screaming people, grabbing at him for some solace.
He was a doctor, not a miracle-worker. He could only do so much. The disease ripped its way through the lives of many, and it didn't help that the medicine they had took three days to completely take effect. McCoy was almost ripping his hair out after twelve different people died at the same time that he was hypospraying them. It was horrible.
He felt like he was trying to give pies to starving children with no time on his hands. There were a couple other Federation ships on the way, but it would take a while before they arrived. Just more time for more deaths.
The thing that probably got to McCoy the most was that these deaths didn't even hit him very hard. He felt like after seeing Spock on the biobed with his internals spilling out and blood all over, he had gone through the worst. Spock was hardly even his friend, if they could be considered as such. Still, it was like this pandemic was just background noise. He felt like crap to think so, but he couldn't help it. Nothing amounted to how he felt when Spock was going to die, and how he had nothing to do about it. That uselessness just killed McCoy inside.
So he kept working. He kept jamming medicine into these people, reading their vitals, and making sure that they wouldn't die on the spot. Everyone else was working so diligently, and he was thankful to have them. He hoped to save as many lives as possible.