This case illustrates something that doctors have long known — that animal diseases probably cause most of the microbes that infect human beings. A Scottish study reported that 61 percent of human infectious diseases originated with animals, as do 75 percent of all "emerging diseases" — the ones that have appeared recently.

Some examples:

  The first human AIDS cases may have been transferred from chimpanzees.

  West Nile virus moved from birds to mosquitoes to people. It is now found in 44 states.

   SARS, sudden acute respiratory syndrome, may have come from civets, a breed of wild cat, in China's Guangdong province.

   Even measles may have been transmitted from animals 5,000 years ago.

"Remember, we're not the dominant life form on Earth," said Dr. Daniel Shapiro, an associate professor of medicine at Boston University. "Bacteria and other infectious agents are, really.

"Because this is novel, it's something we haven't really heard about," said Shapiro. "For a lot of people this is very, very scary."

The good news, says the CDC, is that monkeypox does not seem to be terribly dangerous or very efficient at passing from one person to another. All the suspected victims so far came in direct contact with infected animals, and in some cases were bitten by them.