A five-year-old boy lay in bed burning with fever, his right groin swollen. Suspecting that this was no ordinary disease, his doctor took a sample of fluid from the swollen gland, started home to examine it. But near the house he saw something that confirmed his terrible suspicions: a batch of dead squirrels.
The boy had the plague—the same Black Death that swept over Europe in the 14th Century, killing a fourth of the population. This case occurred in California last month. The boy caught the disease from fleas which carried the plague bacilli (Pasteurella pestis) from sick squirrels. No...