Mortimer carefully put his other experiments away (which took a bit of time) before taking Rabastan's blood sample from his pocket. He was still very confused about why they had his disease. They had all been vaccinated. This should not have occurred.
He poured a sample of the blood into a small dish, corking the rest in the vial and putting it away to use later. He did a few simple spells that produced no result he liked. Then he spoke a charm that magnified the inner structure of the blood.
As he looked, he was fascinated by what he saw. The vaccine he's made was there, working perfectly. But there was also the virus. Not as he originally made it... but changed to completely bypass the vaccine. "It has mutated somehow... something I did not plan on," he said to himself.
He broke the charm and quickly wrote a letter to Rabastan. He sent it off, then took the blood in the dish, extracting the mutated virus and beginning work on a new vaccine.