The country is still stung by the memory of the SARS virus outbreak, which wreaked havoc on the population and business environment in 2003.

Few situations more dramatically illustrate the salience of science to policy than an epidemic. The relevant science takes place rapidly and continually, in the laboratory, clinic, and community. In facing the current swine flu (H1N1 influenza) outbreak, the world has benefited from research investment over many years, as well as from preparedness exercises and planning in many countries. The global public health enterprise has been tempered by the outbreak of severe acute respiratory syndrome (SARS) in 2002–2003, the ongoing threat of highly pathogenic avian flu, and concerns over bioterrorism. Researchers and other experts are now able to make vital contributions in real time. By conducting the right science and communicating expert judgment, scientists can enable policies to be adjusted appropriately as an epidemic scenario unfolds.

So, is the great swine flu scare of 2009 just a big overreaction? The answer, according to public health and infectious disease experts, is no. But the world has been riding what might feel like a roller coaster, set in motion, they say, by the emergence of a menacing pathogen at a time when humanity has never been more primed to fight back. "We've been getting ready for something like this for years," said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "And then this comes along, and all of a sudden the alarm goes off that says: 'Oh, my God, it's here.' " That alarm activated a network of disaster plans put in place after seemingly disparate crises and threats including Sept. 11, the anthrax letters, SARS, Hurricane Katrina and the ominous avian flu virus, which has been skulking around Asia and other parts of the world for several years.

Evolutionary biology may sometimes seem like an arcane academic pursuit, but just try telling that to Gavin Smith, a virologist at Hong Kong University. For the past week, Dr. Smith and six other experts on influenza in Hong Kong, Arizona, California and Britain have been furiously analyzing the new swine flu to figure out how and when it evolved. The first viruses from the outbreak were isolated late last month, but Dr. Smith and his colleagues report on their Web site that the most recent common ancestor of the new viruses existed 6 to 11 months ago. “It could just have been going under the radar,” Dr. Smith said. The current outbreak shows how complex and mysterious the evolution of viruses is. That complexity and mystery are all the more remarkable because a virus is life reduced to its essentials. A human influenza virus, for example, is a protein shell measuring about five-millionths of an inch across, with 10 genes inside. (We have about 20,000.) Some viruses use DNA, like we do, to encode their genes. Others, like the influenza virus, use single-strand RNA. But viruses all have one thing in common, said Roland Wolkowicz, a molecular virologist at San Diego State University: they all reproduce by disintegrating and then reforming. A human flu virus, for example, latches onto a cell in the lining of the nose or throat. It manipulates a receptor on the cell so that the cell engulfs it, whereupon the virus’s genes are released from its protein shell. The host cell begins making genes and proteins that spontaneously assemble into new viruses. “No other entity out there is able to do that,” Dr. Wolkowicz said. “To me, this is what defines a virus.” The sheer number of viruses on Earth is beyond our ability to imagine. “In a small drop of water there are a billion viruses,” Dr. Wolkowicz said. Virologists have estimated that there are a million trillion trillion viruses in the world’s oceans.

Sorting through the “marquee flus” of recent years — SARS, avian flu and now swine flu — is complicated. The three come from different viruses and carry different kinds of danger, depending on ease of transmission and virulence. As a benchmark, the deadliest influenza pandemic in the past century, the Spanish influenza of 1918 to 1919, had an estimated mortality rate of around 2.5 percent but killed tens of millions of people because it spread so widely. The new swine flu cases are caused by an influenza strain called H1N1, which appears to be easily passed from person to person. Mexican health authorities have confirmed 149 deaths from that flu and are investigating the illnesses of 1,600 people, and the United States, Canada, Spain and other countries have confirmed or are investigating cases. But doctors have little information yet on the mortality rate, as there is no reliable data on the total number of people infected. Reports from the United States suggest that some cases may be mild and therefore may go undetected — allowing the disease to spread further. Flu experts are trying to determine if this year’s flu shots, which contain H1N1 strain, offer any protection. In contrast, the lethal avian flu that has kept world health authorities anxious for years is caused by H5N1 influenza virus. It has killed 257 of the 421 people who have contracted it, or 61 percent. But it has shown very little ability to pass from person to person, mainly infecting poultry, and some experts have suggested that there may be something about the H5N1 virus that makes it inherently less transmissible among people. SARS — severe acute respiratory syndrome — is both easily spread and virulent. In the 2003 outbreak in Hong Kong, it killed 299 of the 1,755 people it infected there, or 17 percent. The lessons learned from SARS did not go to waste in Hong Kong. While Mexico struggles to confirm cases of swine flu and sends samples to the United States, Hong Kong is already performing swift genetic tests on patient samples and will have laboratories doing so at six local hospitals by Thursday. Tens of thousands of doctors and nurses, including retirees and those with medical training who have moved to other occupations, are tracked on databases and ready to be mobilized. Contingency plans are ready to keep public transport, electricity, food supplies, telecommunications and other vital services running even if large numbers of people fall ill. And at a time when many hospitals in the United States are already at full capacity and keep few extra beds in reserve, Hong Kong has 1,400 beds in respiratory isolation units, mostly built over the past six years for fear that bird flu or SARS would become a serious problem, and 15 times as many beds as the territory needs on an everyday basis.

Researchers at the University of North Carolina at Chapel Hill and Vanderbilt University Medical Center have synthetically reconstructed the bat variant of the SARS coronavirus (CoV) that caused the SARS epidemic of 2003. The scientists say designing and synthesizing the virus is a major step forward in their ability to find effective vaccines and treatments for any strain of SARS virus that might affect humans in the future.

In 430 B.C., a new and deadly disease—its cause remains a mystery—swept into Athens. The walled Greek city-state was teeming with citizens, soldiers and refugees of the war then raging between Athens and Sparta. As streets filled with corpses, social order broke down. Over the next three years, the illness returned twice and Athens lost a third of its population. It lost the war too. The Plague of Athens marked the beginning of the end of the Golden Age of Greece. The Plague of Athens is one of 10 historically notable outbreaks described in an article in The Lancet Infectious Diseases by authors from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. The phenomenon of widespread, socially disruptive disease outbreaks has a long history prior to HIV/AIDS, severe acute respiratory syndrome (SARS), H5N1 avian influenza and other emerging diseases of the modern era, note the authors. "There appear to be common determinants of disease emergence that transcend time, place and human progress," says NIAID Director Anthony S. Fauci, M.D., one of the study authors. For example, international trade and troop movement during wartime played a role in both the emergence of the Plague of Athens as well as in the spread of influenza during the pandemic of 1918-19. Other factors underlying many instances of emergent diseases are poverty, lack of political will, and changes in climate, ecosystems and land use, the authors contend. "A better understanding of these determinants is essential for our preparedness for the next emerging or re-emerging disease that will inevitably confront us," says Dr. Fauci. "The art of predicting disease emergence is not well developed," says David Morens, M.D., another NIAID author. "We know, however, that the mixture of determinants is becoming ever more complex, and out of this increased complexity comes increased opportunity for diseases to reach epidemic proportions quickly." For example, more people travel more often over greater distances and in less time now than at any time in the past. One consequence of the increased mobility in the modern age can be seen in the 2003 outbreak of the novel illness SARS, which rapidly spread from Hong Kong to Toronto and elsewhere as infected passengers traveled by air.

At least 170,000 Americans die each year from infectious diseases, and that number could increase dramatically during a major disease outbreak. That dire news was delivered in a report,Germs Go Global: Why Emerging Infectious Diseases Are a Threat to America, released Wednesday by the Trust for America's Health (TFAH) ... Emerging diseases such as a potential bird flu outbreak or another new disease such as severe acute respiratory syndrome (SARS).Methicillin-resistant Staphylococcus aureus (MRSA), which is the sixth leading cause of death in the United States. More than 90,000 Americans have been infected by MRSA.Hepatitis C. About 3.2 million Americans have hepatitis C infections, which account for about $15 billion a year in health-care costs.HIV/AIDS, which affects about 1.2 million Americans. Last year, U.S. spending on HIV/AIDS-related medical care, research, prevention and other activities was $23.3 billion.Re-emerging diseases, such as measles, mumps and tuberculosis, which were thought to be nearly eliminated in the United States.