THERE has been a flurry of recent attention over two novel infectious agents: the first, a strain of avian influenza virus (H7N9) in China that is causing severe respiratory disease and other serious health complications in people; the second, a coronavirus, first reported last year in the Middle East, that has brought a crop of new infections. While the number of human cases from these two pathogens has so far been limited, the death rates for each are notably high.

Alarmingly, we face a third, and far more widespread, ailment that has gotten little attention: call it “contagion exhaustion.” News reports on a seemingly unending string of frightening microbes — bird flu, flesh-eating strep, SARS, AIDS, Ebola, drug-resistant bugs in hospitals, the list goes on — have led some people to ho-hum the latest reports.

Some seem to think that public health officials pull a microbe “crisis du jour” out of their proverbial test tube when financing for infectious disease research and control programs appears to be drying up. They dismiss warnings about the latest bugs as “crying wolf.” This misimpression could be deadly.

It’s important to understand our relationship with the microbial world. Most microscopic organisms benefit humans, other organisms or the environment in some way — for example, they help us digest our food and keep bad bugs in check.

At the same time, we are never far away from one of the 1,400 kinds of disease-causing microbes that are capable of infecting people; many infect animals, too. Of these microbes, known as pathogens, about 500 can be transmitted from humans to other humans. And around 150 of them can cause epidemics — rapidly spreading outbreaks of serious, sometimes life-threatening, disease.

Each pathogen has its own “footprint” (or potential footprint) on our human health and social, political and economic landscapes. Far too often the public — and policy makers and journalists — confuse those infectious diseases that can be life-threatening for a limited number of individuals with those that can cause widespread damage to society as a whole.

A disease in the former category is “flesh-eating strep” (invasive group A streptococcal disease). Approximately 9,000 to 11,500 cases are recognized each year in the United States, and about 1,000 to 1,800 of these patients die. When outbreaks of this type occur in this country, particularly if they affect schools or day-care centers, they generate front-page news and widespread concern.

Conversely, last year worldwide 1.7 million of the 34 million people infected with H.I.V. died from AIDS. There was little front-page news coverage about these cases. Nor was there much coverage last year of the estimated 1.5 million tuberculosis-related deaths, of the 1.1 million young children who died of infectious diarrheal illness, or of the 825,000 deaths from malaria. Infectious diseases like these plague the world but, because they don’t occur in our backyard, they remain relatively invisible to Americans.

In the case of the two latest threats — the H7N9 influenza virus and the new coronavirus — the number of infected people is small, and the infections are occurring thousands of miles away from the United States. Yet we should be seriously concerned about both.

Diseases like H7N9 influenza and the new coronavirus are different from noninfectious causes of serious illness and death — and even most microbial causes of disease. They can kill large numbers of people quickly and simultaneously around the world. The 1918 flu pandemic killed an estimated 50 million people worldwide in less than 18 months. The 2003 SARS pandemic, while more limited, resulted in more than 8,275 cases and 775 deaths.

Why does this suddenly happen? Both animal influenza and coronaviruses normally infect animals, not humans. But when these viruses undergo very specific genetic changes that occur as a result of everyday microbial evolution, we have a whole new ballgame; one that is largely played by their rules and on their schedule. Now a virus that once could infect only animals and maybe very rarely infect humans is readily transmitted by people to other people. You could get infected just by breathing shared air with the airplane passenger next to you, or by standing next to the wrong person in an elevator or even by lying next to your sleeping mate. We call this respiratory transmission.

Consider how quickly the H1N1 influenza virus spread in 2009: within the first month of that pandemic, the virus had infected people in at least 42 countries. The only thing keeping these viruses from becoming pandemic killers is their genetics. With few exceptions, all the current human H7N9 and coronavirus cases represent sporadic animal-to-human transmission. But if these viruses continue to spread in their respective animal reservoirs, repeated transmissions of the viruses to humans may lead to the genetic changes that will make either virus readily transmitted by humans to humans. Add in the fact that humans have little to no natural immunity to these viruses, and we could have the next pandemic.

Our public health tools to fight these viruses are limited. We have no vaccines or effective drugs readily available to stop or treat the new coronavirus in the Middle East. And while we have influenza vaccines, my colleagues and I have detailed in an article this week in the Journal of the American Medical Association why we most likely will have limited global impact on an H7N9 pandemic with our current outdated influenza vaccine technology.

In short, we won’t be saved by vaccines if a pandemic emerges from these two new threats. At best, in the case of H7N9, we can only hope that vaccines can help somewhat.

The toll is economic, not just human. Studies have shown that a severe global pandemic, caused by viruses like influenza or coronavirus, could bring the global economy, which is ever reliant on global communications and transportation networks, to its knees. When people are too sick or too afraid to work, borders are closed and global supply chains break, and trade falls. Over months, the economic costs could send the world into recession.

Are either H7N9 or coronavirus pandemics inevitable? We don’t know. But each time one of these viruses infects a human or even another mammal, it’s one more throw at the genetic roulette table.

To reduce the odds of a pandemic, China and the Middle Eastern countries where these viruses are now circulating in animals must do everything they can to identify the animal sources and use every tool they have to eliminate the spreading of the disease. To cull millions of apparently healthy chickens or other domestic animals is not easy, but it is essential.

The world as whole must invest in a new generation of effective influenza and coronavirus vaccines. They are the ultimate insurance policy against similar future emerging viruses. These viruses may seem far away, but tomorrow they could be at America’s doorstep.

Michael T. Osterholm, an epidemiologist, is a professor of environmental health sciences in the School of Public Health, and the director of the Center for Infectious Disease Research and Policy, at the University of Minnesota.