"In several ways. The Law of Simple Reactions also applies, you know." And seeing the blank look on Franklen's face, he added: "When a number of reactions are possible, the one that requires the least energy will happen first. That's saying that the most likely will happen first. A few hundred years ago, so many people died of typhoid, yellow fever, and smallpox that a more complex disease like meningitis or polio seldom got beyond the normal case frequency. When the more prevalent—the more likely diseases—were stopped, we could have polio plagues. It's probably been here for a long time, killing its quota every year, but never noticed because of other, more devastating affairs."
"I think I understand."
"Why did you ask?" asked the doctor.
"Well, I was there, you know. It was rather devastating, though it didn't kill off very many."
"It isn't that type," said Shield. "Which is another factor in its not being noticed. The early symptoms are dysentery, not violent, but definitely weakening. The secondary symptoms are similar to influenza in a mild form. The whole thing just takes all the energy out of the system and leaves you weaker than a kitten for about twenty days. After which you can get up and go again, though somewhat rockily. It's a one-shot affair, luckily. The body builds up an immunity to the bug, and once you go through with it, you're safe from then on—though upon re-exposure, you can act as a carrier."
"O.K., doctor. I was just wondering because I was rather close to it."
"You didn't catch it?"
"Nope. Not yet, anyway."
"And you were in the plague area?" demanded Dr. Shield.
"For several days with Commissario Hohmann."