You can hardly pick up a magazine or turn on the TV these days without hearing about viruses. Dustin Hoffman battles them in the film “Outbreak.” Richard Preston’s “The Hot Zone” is a best-seller. AIDS is regularly in the headlines. In Zaire a whole city is quarantined to contain an outbreak of Ebola.

It’s amazing that something so small could be such big news, but that’s the nature of viruses. One scientist has even called them “humanity’s only real competitors for dominion of the planet.”

Russian botanist Dmitri Iosifovich Ivanovsky discovered viruses a century ago. He was looking for the bacteria he assumed caused tobacco mosaic disease. Instead he found that even when all bacteria were strained out of infected tobacco sap, it could still pass on the disease. Something smaller than a bacteria had to be the infectious agent, something he called a virus (from the Latin word for “poison”). Scientists learned more about them over the next few decades, but nobody actually saw one until the electron microscope came along in the 1940s.

Viruses vary greatly in size, shape and complexity, but basically they consist of a single molecule of DNA or RNA–genetic material–sheathed in protein. By itself, a virus is inert; it doesn’t eat, breathe or reproduce, and therefore can’t really be considered alive. But every virus is capable of infecting a specific host cell. Put it in contact with that host cell, and the virus injects its DNA or RNA into the cell and takes it over. The infected cell begins churning out new copies of the virus, instead of doing whatever it’s supposed to be doing.

Sometimes the host cell dies as a result of the infection, sometimes it survives; whichever, within a matter of hours a single infected cell produces anywhere from hundreds to thousands of new viruses, each of which can infect a new cell.

Among other diseases, viruses cause AIDS, yellow fever, smallpox, polio, mumps, German measles, chicken pox, encephalitis, hepatitis, shingles, herpes, rabies–and, of course, influenza and the “common cold,” itself caused by any one of a couple of hundred viruses, which is one reason we’ll probably never have a cure for it.

There may be lots of other nasty viruses lurking around the world just waiting to get hold of our nice warm cells–and we’re making it easier and easier for them. Historically, certain viruses have been limited to specific ecological niches, confined to the same habitat as their animal hosts. Today, however, people are pushing deeper and deeper into the last remaining wildernesses, especially tropical rain forests, and in the process, are being exposed to new viruses. The ease of international travel means that diseases can span oceans within hours. Similarly, many viral diseases, such as Dengue fever (the most rapidly spreading infectious disease in the world) are carried by insects, and insects, too, can hitch a ride on planes and boats.

AIDS has probably existed for centuries in monkey populations in Africa, but it only emerged this century as humans began to come in closer contact with infected monkeys (and possibly due to a mutation; another charming characteristic of viruses is that they’re constantly evolving, which in itself occasionally gives rise to new diseases, or new versions of old ones) Ebola, too, though it’s probably been around just as long, burst into our awareness in 1976, killing almost 300 people in Zaire. With a fatality rate of 80 to 90 percent, and particularly nasty symptoms (it essentially liquefies certain internal organs), and because of all the books and movies, you might call Ebola the “disease du jour:” it’s the one everyone’s talking about right now.

But lest you think new viral diseases can only arise in the tropics, let’s not forget hantavirus pulmonary syndrome, which killed several people in the southwest U.S. and has since been found to be caused by a virus, carried by mice, that’s widespread across North America, and Lyme disease, which first came to prominence in New England and is caused by a virus carried by ticks.

These emergent diseases–and several others–have appeared within the last two decades. Who knows what other diseases lurk in remote parts of the world, every bit as frightening as Ebola or AIDS, every bit as hard to treat?

In the battle for dominion of the Earth, we’re pitting our vaunted intelligence against inert specks of protein-coated nucleic acid–and right now, the inert specks are winning.

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