In recent decades, emerging diseases have cast a wide net of fear. But where do new infections come from, and how do they break into human populations?

The answer may lie in what they have in common. Two-thirds of all infectious diseases, including those mentioned above, spread to people from other animals. Some scientists say these maladies -- known as zoonotic diseases -- are a growing threat.

Case in point: monkeypox. There have been 87 suspected cases of monkeypox in six U.S. states since the outbreak began this spring. According to the Centers for Disease Control, all cases appear to have been the result of contact with captive prairie dogs or other pets. The prairie dogs were probably infected by a Gambian giant rat imported from West Africa, where the disease is endemic.

Monkeypox is milder than its infamous cousin, smallpox, and probably can't be spread from person to person. But smallpox only has one natural host: us. That's why it could be eradicated. Monkeypox should be considered a "harbinger of things to come," says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

The booming trade in exotic pets -- coupled with a globalized, ever-growing human population -- is an invitation to pathogens looking for homes, says Osterholm.

How to attract a germ
In Darwinian terms, an outbreak is at once a defeat and a great achievement. A disease that kills a thousand people can beget a million bugs. It's true for viruses, bacteria, fungi, and parasites: It sometimes takes but a subtle tweak for a pathogen to match a new environment, or a new host.

No surprise, then, that environmental changes -- deforestation, climate change, the transit of populations -- often invite new infections.

Zoonotic diseases emerge for many reasons, says Osterholm. But three top the list.

In 1900, there were 750 million people on Earth. Now there are 6.2 billion. That staggering growth has pushed people deep into previously untouched corners of the world, where people encounter new animals and their indigenous pathogens.

Partly because of this population growth, farmers now raise great numbers of livestock in crowded spaces. And new animals are regularly added to the food supply as traditional stocks are depleted. What's more, in many countries people and animals regularly intermingle in large urban markets. Mixing people and animals together, in large numbers and tight spaces, is a recipe for disease.

Last is the trend that now hogs the spotlight: Wild animals can be cute, interesting, and certainly attention-gathering. So naturally, Americans want them as pets. This demand ensures the global trade in exotic animals responsible for the recent monkeypox outbreak in the Midwest.

"Since time immemorial, animals have been the source of infectious disease in humans," says Beatrice Hahn, a virologist at the University of Alabama, Birmingham. "Going back to prehistoric times, it's clear that when humans change their impact on the environment, or change farming practices -- every time, it opens an opportunity that microbes will invariably take advantage of."

It's hard to say for sure whether zoonotic diseases are becoming more common, says Hahn. To do so would require decades of reliable epidemiological records from around the world. That kind of data just isn't available, Hahn says. Still, she says, "there has been a new infectious disease, or the emergence of a known infectious disease, every single year for the past two decades. So even if the number isn't increasing, it is high."

For most zoonotic pathogens, people are dead-end hosts. An animal reservoir provides a permanent home, and people are only infected incidentally, as appears to have been the case with monkeypox. But sometimes, a virus "jumps" to people from other animals and starts to thrive in human populations. The virus that causes SARS, for example, infected civet cats and then the people who handled them in a market that supplied restaurants in China's Guandong province. (Zoonoses are distinct from vector-borne diseases, like malaria, that are spread from person to person by arthropods like ticks or mosquitoes.)

SARS pales in comparison to the headline-makers of history.

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Terry Devitt, editor; Sarah Goforth, feature writer for this week and project assistant; S.V. Medaris, designer/illustrator; Amy Toburen, content development executive

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