PHOTOGRAPH BY CHARLIE NEIBERGALL/AP

Which epidemic has caused Americans greater anxiety: the Ebola outbreak, which began last year in Africa and has killed thousands of people there, or the strain of influenza that has now infected poultry in twenty-one states?

The answer is obvious. Ebola, which was responsible for two deaths in the United States, caused a level of unfettered hysteria hardly seen here since Senator Joseph McCarthy accused half the country’s creative class of holding membership in the Communist Party. The bird flu, although it has now caused the deaths of more than forty-five million chickens, devastated the American poultry industry, doubled the price of most eggs, and is by far the worst such outbreak in U.S. history, somehow doesn’t seem as menacing to people in this country. No doubt that is in part because the virus has not killed a single person. It doesn’t even infect humans. We can only hope that those facts remain unchanged. But we shouldn’t bet millions of lives on it.

How many times does the world have to be threatened with a deadly pandemic that moves from one species to another before people get the point? The message didn’t seem to sink in after the emergence, in 1996, of a previous strain of avian influenza, one that did kill people. Or after SARS, the first serious, easily transmitted disease to emerge in the twenty-first century, swept into Hong Kong, in 2003. Few people seemed afraid of the H1N1 influenza virus in 2009, which, while unusually mild, infected as many as a hundred million Americans and nearly a billion people throughout the world.

Each of those pandemics started in animals, then mutated and became infectious to humans. But vigorous responses by public-health officials, as well as plain luck, limited their impact. “If H1N1 had been more virulent, it would have killed millions of people,” the Stanford biologist Nathan Wolfe told me at the time. “Maybe tens of millions. Once it got out there, that thing burned right through the forest. We caught an amazingly lucky break, but let’s not kid ourselves. Luck like that doesn’t last.”

Most viruses do not jump from one species to another. In 1983, for example, a strain of avian influenza raced so rapidly through the Pennsylvania poultry population that health officials there were forced to slaughter nearly every chicken in the state. That outbreak stopped with chickens. But the latest avian influenza virus, which was brought to the U.S. last year by wild birds migrating across the Pacific flyways, is not like most others. It soon infected wild waterfowl—and then even started to kill domesticated birds.

“Now we have a highly pathogenic virus in wild birds that jumped to domestic” flocks, David Suarez, a research leader at the U.S.D.A.’s Agricultural Research Service, told the Minneapolis Star Tribune on Monday. “This is an all-hands-on-deck situation.”

It would be hard to overstate the importance of the poultry industry in America and many other countries. Often, little more than a month will pass from the time a chicken is hatched until it appears on your dinner plate. There are few other sources of protein so readily available. On Monday, the Secretary of Agriculture, Tom Vilsack, said that the government could end up spending as much as five hundred million dollars to deal with the outbreak. (The private sector will spend much more than that.) Vilsack noted that domestic egg production would fall in 2015 for the first time since 2008. He also stressed the fact that, unless we devote more resources to curtailing emerging diseases, the situation can only get worse.

“We used to think we had outstanding biosecurity in poultry,” Michael T. Osterholm told me. Osterholm, an expert on biosecurity, is the director of the Center for Infectious Disease Research and Policy, at the University of Minnesota. “But, except for the outbreak in 1983, which was stopped quickly, we have never been tested before.” Like most infectious-disease researchers, Osterholm says that we need better systems to screen for viruses as they emerge from animals, and more money devoted to developing effective drugs and vaccines.

The avian influenza virus, as it is constituted now, does not attach to receptors in the human throat. But many of the largest hog-production facilities in the United States are located in the upper Midwest—often on the same farms where millions of chickens and turkeys are bred. That makes for a particularly ominous convergence: epidemiologists consider pigs an ideal mixing vessel for human and animal flu viruses, because the receptors on their respiratory cells are similar to ours.

None of this is news. Nor is the fact that we live in a world where species mix constantly, and viruses can cross the Earth in a day. MERS, or Middle Eastern Respiratory Syndrome, is an example. The virus, which appears to have jumped from camels to humans, is similar in many ways to SARS, and its earliest symptoms resemble those of a common cold. It can be deadly, though, and there is no cure.

Until recently, the bulk of MERS cases have occurred in the countries of the Arabian Peninsula. But, in the past month, the epidemic has spread to South Korea—brought unwittingly, it appears, by a man travelling from the Middle East. “This is how viruses work in the modern world,” Osterholm said. “And though it’s a tragedy, if it had gone to Mombasa instead of Seoul, we would really have a crisis on our hands. I don’t want to say that is inevitable. But, unless we work far harder to contain these viruses when we first learn of them, this kind of global transmission can only get worse.”