The deadliest plague in human history was the influenza pandemic of 1918, which may have killed as many as 100 million people worldwide, according to recent estimates. Modern flu strains tend to spare young healthy adults, but every few decades a strain arises that can kill people in the prime of life. In 1918, more than a quarter of all Americans fell ill.

For millions across the globe, what started as a runny nose and a sore throat ended days later with bleeding from the ears and nostrils and into the lungs. Victims literally drowned in their own blood. Their corpses—tinged blue from suffocation—were said to have been stacked like cordwood outside the morgues as cities ran out of coffins.

Where did this disease come from? Doing brilliant medical detective work, Yale and Scripps researchers examined the corpses of 1918 flu victims discovered frozen in the Alaskan permafrost. The corpses had the blood-filled lungs typical of this disease. And investigators recently pieced together the genetic makeup of the virus by examining tissue samples from the bodies. The disease came from bird flu.

We now know that bird flu is the original cause of all of these so-called human influenza "type A" viruses. Although the viruses can affect a wide range of animals including pigs, horses and wild birds, the initial source seems to be domesticated fowl such as chickens and turkeys.

Large commercial poultry operations provide an ideal spawning ground for new pandemic strains. On factory farms that confine birds raised for meat, thousands of chickens are clustered in huge sheds. Because they live in their own manure, the virus can be excreted in the feces and then breathed in or swallowed by thousands of other birds, allowing the virus to circulate rapidly and repeatedly. With so many birds passing the virus back and forth, giving it a chance to mutate each time, low virulence strains of influenza can turn into deadly ones.

Over the last few decades meat and egg consumption has exploded in the developing world, leading to industrial-scale commercial chicken farming and mass animal transport, the perfect storm environment for the emergence and spread of influenza super-strains. The World Animal Health Organization blames changes in the global poultry industry, such as shorter production cycles and greater animal densities, for the increased risk of epidemics. The U.N. Food and Agriculture Organization's Director of Animal Production and Health, Dr. Samuel Jutzi, agrees. "As soon as you have that many animals in one spot you are likely to get into trouble with disease," he says.

The World Health Organization also blames the latest bird flu outbreak in southeast Asia on "intensive poultry production." The stress of intensive confinement alone on the birds' immune systems increases the risk that factory farms will become the breeding ground for the next global pandemic.

"Chickens used to live in our backyards," explains senior Thai public health officer Charun Boonyarithikarn. "They didn't travel much. Now, throughout the world, farms have become factories. Millions of chickens are shipped huge distances every day. We can't stop every chicken or duck or pig. And they offer millions of opportunities for pathogens to find a niche."

Although more than 100 million birds are dead from the latest outbreak, fewer than 100 people in southeast Asia have died of the current strain of bird flu. Nearly all of the human deaths have involved people who lived or worked with poultry, poultry meat, or eggs. U.S. Department of Agriculture researchers tested thigh and breast meat in chickens and effectively proved in a study published in March 2005 that chicken meat from infected birds can indeed be a source of infection.

While UN officials have urged people to stop drinking duck's blood and eating "tiet canh," congealed duck blood pudding, the fear is that once the bird flu virus infects enough human hosts, it can mutate into a form that can be easily transmitted person to person—through a sneeze or a handshake—triggering the next global pandemic. The head of the World Health Organization recently told a press conference, "The world is now in the gravest possible danger of a pandemic."

Dr. Michael Osterholm, director of the U.S. Center for Infectious Disease Research and Policy, may be the nation's top expert in bird flu. He attempts to describe the potential catastrophe, suggesting that people consider the devastation of the recent tsunamis in South Asia. "Duplicate it in every major urban centre and rural community around the planet simultaneously, add in the paralyzing fear and panic of contagion, and we begin to get some sense of the potential of pandemic influenza," he said. "An influenza pandemic of even moderate impact will result in the biggest single human disaster ever—far greater than AIDS, 9/11, all wars in the 20th century and the recent tsunami combined. It has the potential to redirect world history as the Black Death redirected European history in the 14th century."

Humanity's lust for cheap meat not only leads to the suffering and deaths of billions of animals every year directly, but threatens the health of our planet and may threaten our personal health in more ways than we suspected.

Michael Greger, M.D. is The HSUS's Director of Public Health and Animal Agriculture.