So the vector experiments fell to Burton.

They were reasonably simple and straightforward, designed to answer the question of how the disease was transmitted. Burton began with a series of cages, lined up in a row. Each had a separate air supply; the air supplies could be interconnected in a variety of ways.

Burton placed the corpse of the dead Norway rat, which was contained in an airtight cage, alongside another cage containing a living rat. He punched buttons; air was allowed to pass freely from one cage to the other.

The living rat flopped over and died.

Interesting, he thought. Airborne transmission. He hooked up a second cage with a live rat, but inserted a Millipore filter between the living and dead rat cages. This filter had perforations 100 angstroms in diameter—the size of a small virus.

He opened the passage between the two cages. The rat remained alive.

He watched for several moments, until he was satisfied. Whatever it was that transmitted the disease, it was larger than a virus. He changed the filter, replacing it with a larger one, and then another still larger. He continued in this way until the rat died.

The filter had allowed the agent to pass. He checked it: two microns in diameter, roughly the size of a small cell. He thought to himself that he had just learned something very valuable indeed: the size of the infectious agent.

This was important, for in a single simple experiment he had ruled out the possibility that a protein or a chemical molecule of some kind was doing the damage. At Piedmont, he and Stone had been concerned about a gas, perhaps a gas released as waste from the living organism.

Yet, clearly, no gas was responsible. The disease was transmitted by something the size of a cell that was very much bigger than a molecule, or gas droplet.

The next step was equally simple—to determine whether dead animals were potentially infectious.

He took one of the dead rats and pumped the air out of its cage. He waited until the air was fully evacuated. In the pressure fall, the rat ruptured, bursting open. Burton ignored this.

When he was sure all air was removed, he replaced the air with fresh, clean, filtered air. Then he connected the cage to the cage of a living animal.

Nothing happened.

Interesting, he thought. Using a remotely controlled scalpel, he sliced open the dead animal further, to make sure any organisms contained inside the carcass would be released into the atmosphere.

Nothing happened. The live rat scampered about its cage happily.

The results were quite clear: dead animals were not infectious. That was why, he thought, the buzzards could chew at the Piedmont victims and not die. Corpses could not transmit the disease; only the bugs themselves, carried in the air, could do so.

Bugs in the air were deadly.

Bugs in the corpse were harmless.

In a sense, this was predictable. It had to do with theories of accommodation and mutual adaptation between bacteria and man. Burton had long been interested in this problem, and had lectured on it at the Baylor Medical School.