Over the past two weeks, the global public-health establishment has treated the world to the unseemly spectacle of what is ultimately another pandemic false alarm. What happened?
In late April, we received reports of deaths, predominantly in younger people, from a new strain of influenza in Mexico. This is how new influenza pandemics are expected to start, so this information was initially very concerning.
The World Health Organization upgraded its pandemic alert to phase 4. Public health authorities in Canada and other countries began to launch pandemic plans. News stories about "swine flu" and an unfolding pandemic dominated front pages and television pundits brought dire warnings.
Then we began to get better information. The initial Mexican death count - fewer than 150 - was tiny compared with the 20,000 to 200,000 deaths that could be expected there during an influenza pandemic. Most importantly, the daily count was not rising exponentially. Returning travellers were not becoming seriously ill.
Notwithstanding, on April 29, WHO director-general Margaret Chan raised the alert level to phase 5 and declared that it is "all of humanity that is under threat from a pandemic." The Public Health Agency of Canada advised against travel to Mexico.
It now appears that this new virus has been infecting people in Mexico since early March - almost two months. The illness has probably already infected millions of Mexicans and many visitors, but it has been almost uniformly mild. Some Mexicans developed a more severe version, but they were almost certainly only a tiny proportion of the total number of cases.
The perceived pandemic threat is melting away under the scrutiny of better information. It is difficult to predict the future of this new H1N1 influenza virus. It may simply fade away, or it may become established as a new seasonal strain. In any case, it is clearly not a threat to "all humanity."
Many commentators have begun to look for a face-saving exit strategy. Some are warning that the virus could mutate and return stronger in the fall. But there is no scientific basis for this speculation.
This is not the first time that false alarm bells have sounded. In 1976, a highly touted swine flu epidemic failed to spread. Severe acute respiratory syndrome was supposed to cause a devastating pandemic but didn't. Since 2004, we have faced an interminable series of alarms about the imminent danger of H5N1 - bird flu - usually from the same voices that have been so prominent in the past two weeks. But that pandemic hasn't happened either.
There are very real costs to these false alarms. Look at the damage to Mexico's vital tourism industry. In 2003, Canada protested long and hard against the WHO's SARS travel advisory. But the Public Health Agency of Canada barely hesitated before making a similar assault on Mexico. This is wrong.
Frightening people without due cause is inherently bad. Confidence in the credibility of public-health authorities will suffer. Will people still believe us when a real crisis emerges?
The risk of a future influenza pandemic deserves our respect, but it should not cause us to live in fear. Pandemics are not usually catastrophic events. The most recent - 1957 and 1968 - were almost indistinguishable from annual seasonal influenza. The 1918 pandemic was more serious, but the world is a very different place now.
What are the lessons from this latest false alarm?
First and foremost, we should act on the basis of good information and analysis, not fears and preconceptions. There is no shame in backing down from an alarm when our understanding of the evidence changes. Give the public a balanced view as the facts emerge.
Second, our response needs to be commensurate with the threat. The damage done by travel advisories, futile traveller screening and persistent alarmist pronouncements is wholly gratuitous.
And, finally, we need to calm down. Obesity, tobacco addiction, climate change, poverty and nuclear war - these are the real public-health threats to all humanity. Influenza is not.
Richard Schabas was Ontario's chief medical officer of health from 1987 to 1997. Neil Rau is an infectious diseases specialist and medical microbiologist in Oakville, Ont., and a lecturer at the University of Toronto.