The abandoned barn was anything but abandoned. A nest of bees had been growing steadily since the demise of the said building.
At first, it was only a small lump on the wall, but after a few weeks, it had grown considerably. No one really paid much attention to the increase of the bee population in the area, so their numbers grew merrily.
It was inevitable for someone to be stung. First a child, then an adult, then many adults, a crowd of adults. Panic took over the town and within a few weeks it was clear that the sanitary services had to come up from the city to help solve the problem.
The bees were resilient though. No poison, spraying, destroying the hive made them disappear.
As a matter of fact, some of the local residents showed evidence of disturbing mutations. They tended to linger a lot longer around the flowerbeds in the public parks, they felt strangely attracted to that barn and all of them made involuntary humming noises.
The situation was so catastrophic that the whole town was put under a strict, and useless, quarantine.
It took them about a year, but when they finally controlled the epidemic all over the country, it was extremely difficult to set the original bees and the mutated humans apart, except for the oversized dimensions of the latter, of course.