Somewhere out there, somewhere along the way, a single creature got all this started. A pig, presumably. Pig Zero.
Scientists suspect that two influenza viruses common in swine, one rooted in Eurasia and the other in North America, came together in a single cell within a pig. The two viruses exchanged their genes like a couple of kids swapping school clothes. The result was a novel strain of virus, with, according to scientists, two genes from the Eurasian virus and six genes from the North American virus.
The new strain then jumped to humans. Where is unknown. Mexico is a possibility, but so far the virus hasn't been found in any Mexican swine.
All of this is the latest iteration of a phenomenon dating to the dawn of mankind: zoonosis. A zoonotic disease is one that spreads from animals to humans, or vice versa. Bubonic plague came from a bacterium that infects rats and can spread via fleas to humans. HIV is a virus that passed into people from a monkey. Malaria, tuberculosis, rabies, yellow fever and typhoid fever are zoonotic.
And it's a two-way street, as seen recently when a Canadian farmworker infected with the new H1N1 swine flu apparently passed the disease to a herd of pigs. When it comes to influenza, the thoroughfare between Homo sapiens and Sus scrofa — domesticated pigs — is something of a superhighway.
From the perspective of an influenza virus, the receptors on the lungs of a human being — the places where the little spiky knobs on the virus can attach themselves — look very much like the receptors in a pig. A pig's anatomy is so similar in certain respects to a human being's that pig heart valves are routinely transplanted into human heart patients.
"Zoonotic agents don't care whether it's a human or an animal ," said Juergen Richt, a professor of veterinary medicine at Kansas State University.
Thanks to vaccines and antibiotics, the war against infectious diseases seemed to be nearly won by the second half of the 20th century, but the pathogens have shown themselves to be resilient and adaptive. Meanwhile, the human population has grown to more than 6 billion, sustained by billions of farm animals, many raised in close quarters on factory farms, said JoLynn Montgomery, an epidemiologist at the University of Michigan.
"There's more crowding in animals, and more crowding in people, and the crowding is merging," she said. "People are getting diseases from animals more frequently. I'm not sure the diseases themselves are getting worse." Public health measures — careful surveillance of zoonotic diseases — can counterattack the problem, she said.
Zoonotic diseases can also come from wild animals, and new pathogens can emerge as human beings penetrate remote, isolated regions of the planet, said Thomas J. Inzana, a bacteriologist at Virginia Tech. Some exotic pathogens are so "hot" that they can't spread as easily as viruses that are less lethal, he noted: "It doesn't do the pathogen any good to kill its host."
Which is why flu is such a problem: It has essentially co-evolved with people, pigs, birds and other animals. And it's malleable. Influenza is what is known as an RNA virus. Such viruses, mere snippets of genetic material, replicate inexactly, like photocopy machines on the fritz. That sloppiness enables them to evolve rapidly and find new hosts, and makes them a moving target for vaccine makers.
The specific origin of the new flu strain remains a matter of intense investigation. Even the presumption of a Pig Zero is just educated guesswork. The new virus conceivably could have spliced itself together inside a human being or some kind of bird. A pig is the most likely source simply because two ancestral viruses had clear genetic markers of swine-related flu, and a pig is the most likely place for two swine flus to converge, said Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health.
That said, a veterinarian and spokeswoman for the National Pork Producers Council, Jen Greiner, emphatically rejects any presumption that the new strain emerged in a pig: "It has not been found in any pigs in Mexico; it has not been found in any pigs in the U.S.," she says.
Influenza is hardly just a human-and-pig tango. Birds can also get flu. So can horses. But although humans can, in rare cases, catch bird flu, it doesn't then spread from person to person. Humans can't catch horse flu. Dogs, however, can catch horse flu. Why these viruses do what they do and jump where they jump is not well understood, Richt says. He says that's all the more reason for veterinary health research to be on equal funding footing with human medical research. "You need to understand what's happening in the cattle, the pig, the raccoon, to protect the human population," Richt said.
There's a lot a human would recognize in a pig with flu. "The pig feels hot, and it doesn't eat as much," Richt said. If the condition of pigs worsens, "they develop nasal and ocular secretions. And if they go further, they start to have respiratory distress, so they breathe faster, and they can go to coughing, sneezing."
Although factory farming has been a target of much criticism, it has its defenders. Marie Gramer, a veterinarian at the University of Minnesota, said enclosed farm buildings offer "biosecurity" from pathogens carried by wild animals.
"The health now of the collective animal agriculture is better than it was 20, 40, 60 years ago," Gramer said.
Although some of the worst fears about the current swine flu outbreak have subsided — the virus doesn't appear to be as virulent as first thought — the very nature of influenza makes the future of this strain impossible to predict. It will surely evolve further, Pekosz said.
"This is a brand-new virus and a brand-new host," Pekosz said. The process of natural selection will tug the virus in new directions, he said. His scientific prediction — "That gene constellation is probably going to optimize itself to replicate" — strongly suggests that human beings haven't heard the last of this new swine flu.