Plague
The epidemic known to its survivors as The Great Pestilence, and to history as The Black Death, swept across Europe from 1347-1351, leaving more than 25 million people dead. The prevailing theory is that Black Death was an epidemic of bubonic plague, a flea-borne bacterial disease that thrives in rodents. This theory has been called into question in recent years (See "Ring a ring o' roses... in the bibliography). Nonetheless, plague -- one of humankind's oldest infectious diseases -- has left an undeniable stain on human history, says CDC plague expert Ken Gage.

Plague is still endemic to parts of Asia, Africa, and the Americas. It appears periodically, and since its host animals -- rodents -- are likely to be around for a long time, so is the disease. It's worst in places where people live closely together and poverty rages. A 1994 outbreak in Surat, India, left 500 people dead. Considered by some to be a reemerging disease, the plague's geographic range has expanded steadily in the last century. The World Health Organization reports that an average of 547 cases -- and 181 deaths -- occur each year. Thankfully, antibiotics -- when available -- can usually treat the disease successfully.

AIDS
The statistics never fail to startle:

42 million people worldwide -- 38.6 million adults and 3.2 million children -- are HIV positive. About 70 percent of these people live in Sub-Saharan Africa.

The WHO reported that five million new HIV infections occurred worldwide in 2002. That's an average of 14,000 new infections each day. More than 95 percent occurred in developing countries.

More than three million people died from AIDS in 2002.

According to the CDC, 40,000 new HIV infections occur each year in the United States. Of these newly infected people, half are 25 years old or younger.

Beatrice Hahn was a member of the team who first described AIDS as a zoonosis in 2000. The team wrote that HIV-1, the virus responsible for the global AIDS pandemic, first spread to humans from contact with chimpanzees in Africa. And HIV-2, a form of the virus that remains confined to Africa, originated in another African primate species, the sooty mangabey. The jump probably occurred when hunters in the forests of the Congo basin came into contact with monkey blood infected with simian immunodeficiency virus (SIV), a relative of HIV.

A child suffering from AIDS. Photo: USAID

This June, however, the team reported an interesting new twist: The SIV variant that led to HIV-1 appeared even earlier in a species of monkey eaten by the chimps.

"We had always lumped chimps together with all the other primate species that naturally harbor SIV," Hahn says. "This finding showed us that chimps acquired it by predation, which is very similar to how humans acquired it."

The hunting of wild primates for food is still a widespread practice in Africa, Hahn warns. Last year, her team documented the presence of multiple SIV strains in African primates. Hahn worries that new varieties of HIV might emerge where hunters -- and consumers -- of wild primates are exposed to different strains of SIV.

Influenza
In China, where pigs are farmed closely with ducks, the flu viruses native to each species regularly intermingle. As a result, a new form of the virus emerges nearly every year, eventually spreading around the world. One type of influenza usually confers little or no immunity against other types, so every year scientists hustle to make a new vaccine.

The 1918 flu -- the worst pandemic of the disease in recorded history -- killed 40 million people. Scientists say we could be in for another "superflu." Photo: CDC

Sometimes, the flu virus evolves into a more dangerous, or more communicable, disease. That's what happened in 1918, when the insidious "Spanish Flu" killed 40 million people worldwide. More recently, two pandemics involving strains of the virus related to the Spanish Flu arose in 1957 ("Asian influenza") and 1968 ("Hong Kong influenza").

As a matter of probability, scientists say the likelihood of a harsh, fast-spreading flu grows with Asia's burgeoning cities. The CDC has warned that a such an outbreak could strike within ten years, quite possibly on the scale of the 1918 epidemic.

Hantavirus
Europe and Asia have a long history of human disease caused by the hantaviruses, which are naturally carried by mice and other rodents. These viruses can cause kidney failure and sometimes haemorrhagic fever, killing about 10 per cent of victims. Hantavirus was unknown in the US until 1993. That year, the region was struck with heavy rains -- and, as a result, a thriving mouse population. Soon, otherwise healthy, young people began dying in the region's rural communities. The cause of death was a respiratory syndrome caused by virus previously thought not to infect people. Hantavirus, which is spread in the feces and urine of field mice, had probably been around for decades.

Hantavirus, the deadly disease that struck the American southwest in the 1990s, is carried by rodents, especially the deer mouse. Humans can be infected when the accidentally ingest, or inhale, particles from the animals' droppings. Photo: National Biological Information Structure

Ebola
In the spring of 1976, hundreds of cases of a mysterious -- and deadly -- disease began to surface in Africa. Ebola haemorrhagic fever (EHF) is one of the most virulent known diseases, causing death in 50-90 percent of all cases. Scientists think the disease first emerged in African jungles, spreading to people by contact with monkeys. But because the disease kills most of its primate hosts, scientists suspect there is another -- as yet unknown -- natural reservoir. There is no specific treatment or vaccine, and the highly contagious disease has cropped up again in recent years.

Of course, there are some more recent candidates for this grisly list: mad cow disease, West Nile fever, and anthrax.

And there may be more to come.

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