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The first reports about swine flu in Mexico made the disease sound highly lethal. But now, just a couple of weeks later, public-health officials are saying the new H1N1 strain may be no more deadly than plain-old, seasonal flu. Disease experts say the downgrade is not surprising. It's a product of the way most disease outbreaks are detected.
NPR's Jon Hamilton has the story.
JON HAMILTON: Swine flu isn't the first virus to look terrifying early on. Back in 2003, a bug surfaced in Toronto that looked even worse. Rob Fowler is a critical-care doctor at Sunnybrook Hospital and the University of Toronto. He remembers getting a call one night about a man with severe breathing problems. What sticks in Fowler's mind is what he learned about the man's family.
Dr. ROB FOWLER (Sunnybrook Hospital, University of Toronto): A brother had been admitted to hospital earlier in the day and had suffered a respiratory arrest. Another first-degree relative of the patient was in an ambulance en route to another hospital's intensive care unit in the city. The mother had died.
HAMILTON: Fowler says that was the beginning of Toronto's experience with the disease called SARS.
Dr. FOWLER: Over the next weeks to months, we saw hundreds of patients with SARS, and it really shut down not just the medical infrastructure for the city, but really, the financial and social sort of structure of the city while we sort of dealt with this outbreak.
HAMILTON: At first, it appeared that SARS was killing nearly 50 percent of the people who got infected. But as the outbreak progressed, it became clear the true death rate was closer to 5 percent. Fowler says it took a while before doctors started looking for SARS in people who weren't critically ill. And that made the disease look even more frightening than it turned out to be.
Dr. FOWLER: The ones that you notice are the folks that are different because they've gotten very sick. They've come to medical attention, and people really start to notice when deaths occur.
HAMILTON: Something similar had had happened with West Nile virus a few years earlier. The first known cases were people with a rare brain inflammation. But before long, blood tests revealed that lots of infected people had no symptoms or just a mild fever.
Annie Fine, with the New York City Department of Mental Health and Hygiene, says she learned something from West Nile.
Dr. ANNIE FINE (New York City Department of Mental Health and Hygiene): It's very, very difficult to get a handle on how many people are out there who may also have the same infection.
HAMILTON: And Fine says that's been the challenge with swine flu as well. In Mexico, it was the sickest people who got tested first. Frederick Hayden, a flu expert at the University of Virginia, says that was bound to be misleading.
Dr. FREDERICK HAYDEN (Flu Expert, University of Virginia): The hospitalized patients really represent only a fraction of all those affected. And of those who go on to die, represent, really, a small portion, presumably, of all the cases that have happened.
HAMILTON: The mild cases can go undetected for weeks or months. But this time flu experts got a break. They found out that the virus had infected dozens of students from a school in New York after some of them had visited Mexico. Hayden says that let scientists study a large group of flu patients identified outside a hospital.
Dr. HAYDEN: There were no complications or fatalities. So, you know, that kind of experience shows both the dramatic transmissibility of this virus in susceptible populations, but also the fact that the majority of those affected handle it and recover spontaneously, without any specific therapy.
HAMILTON: And that appears to be what's happening even as the number of swine flu cases climbs into the thousands worldwide.
Jon Hamilton, NPR News.
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