The rapid growth of humanity, with a corresponding shrinkage of living space and species diversity, has upset this balance. For example, the world's rain forests contain, by far, the largest diversity of species on Earth, somewhere between 3 and 30 million species of animals and plants. Since all living creatures carry viruses, from a few to many (humans carry at least 100 viruses alone for the common cold, for example) this means rain forests are viral hotbeds.

In the meantime, there are other worries. The flu virus is easily transmissible through airborne particles, but it's not inherently fatal. Usually it kills the weak and elderly. Some emerging viruses are much less discriminating. Some victims of the Four Corners hantavirus were young and in good health. Yet they quickly succumbed once flu-like symptoms appeared.

Muggers, however, are usually visible. Viruses are impossibly small. A typical bacterium is perhaps 1 micron in diameter, about .00004 of an inch wide. Vaccinia virus, one of the largest viruses known, is just one-fourth of a micron in diameter. The polio virus, one of the smallest, is a mere 12 thousandths of a micron wide; 100 million of them packed together only one virus deep could easily fit on the period at the end of this sentence.