Timeline of a pandemic

In early April, a 10-year-old San Diego boy was found to be infected with a novel flu virus. The virus, identified as an H1N1 strain of swine origin, was soon matched to samples from Mexico, which had suffered a series of flu outbreaks beginning in March. Those outbreaks had led to a large number of deaths and hospitalizations. Because of its virulence and the low resistance amid the general population, global and U.S. public health officials quickly recognized that the new virus strain could have pandemic potential.

Here’s a closer look at the progression of this flu outbreak, which did indeed become a pandemic.

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April and May

In Mexico, so-called swine flu continued to spread. In late April, the Mexican government closed the nation’s schools. Eat-in restaurants in Mexico City were shut down to curb transmission. Fear of flu led several cruise-line companies to suspend stops in the country, and Britons were warned to avoid all but essential travel to Mexico.

In the U.S., the disease spread quickly. By the end of April, five states had confirmed cases. A New York City school was the focus of that city’s outbreak after some students returned from spring break trips to Mexico.

The spread of the virus in the Northern Hemisphere continued through May, as seasonal flu cases receded and a 33-year-old woman in Texas died of the disease -- the first U.S. citizen to fall.

Swine flu also crossed oceans, with early cases appearing in Europe (Spain and France), Asia (South Korea and Hong Kong) and the Southern Hemisphere (New Zealand). On April 27, the World Health Organization raised the pandemic threat level to 4, meaning sustained human-to-human transmission. (Phases 1 to 3 are for predominantly animal infections.) Two days later, the alert level was raised to 5, indicating community-level transmission.

In early May, airports in India, China and Singapore began screening international passengers for signs of illness.

On May 6, schools and restaurants reopened in Mexico.

By mid-May, H1N1 flu arrived on mainland China and in Japan. Later in the month, the illness began to show up in South America, with 74 confirmed cases in Chile. On May 29, Egypt ordered the slaughter of 300,000 pigs because of swine flu fears.

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June and July

At the beginning of June, 62 countries had confirmed cases of H1N1 infection. As the world watched, waited and prepared, WHO struggled with raising its pandemic alert to the highest level. One issue that muddied the alert system was that WHO’s pandemic definition is based on spread of the disease, not severity. Although hospitalizations and deaths were being reported, for the most part, the novel H1N1 flu seemed to be a rather mild illness.

Egypt reported its first case on June 8 -- the first on the African continent.

On June 11, WHO moved to level 6 pandemic alert.

As the epidemic continued well past the normal flu season, 3,000 to 4,000 probable cases were reported per week, according to data from the federal Centers for Disease Control and Prevention. An estimated 1 million people in the U.S. had been infected by the end of June.

The big question for the early summer months was: What would happen in the Southern Hemisphere as that part of the globe entered its traditional flu season?

Infectious disease experts watched to determine whether H1N1 swine flu would dominate or whether it would co-circulate with H1N1 seasonal flu and influenza B. They were also on the lookout for signs the novel flu virus was changing in any way -- especially in the severity of illness it caused.

“In terms of spread, we pretty much saw exactly what we were expecting to see,” says Ira Longini, a professor of biostatistics and epidemiology at the University of Washington in Seattle. In Australia and New Zealand, he says, “It’s very clear they had a sharp peak epidemic. They’re on the other side of the peak now.”

In South America, Argentina and Chile had similar epidemics. A recent report at flu.gov says that these four Southern Hemisphere countries had disease activity that was mostly novel H1N1 flu and dropped off after mid-July -- similar to their typical flu seasons.

“Most recently we’ve been seeing a huge increase in Bolivia,” says John Brownstein, a Harvard epidemiologist who co-created HealthMap, a disease-tracking system that compiles news reports and public health data available on the Internet. “Also Venezuela and Colombia.”

In late June, Mexico City offered free health insurance to visitors staying in participating hotels to boost its sagging tourism industry.

The U.S., meanwhile, saw outbreaks at camps and military academies and in jails. Twenty cases were reported in Los Angeles County jails. Community outbreaks also occurred; Orange County reported more than 450 cases in the months of June and July and suffered four deaths in a two-week period. As of July 23, California had 583 hospitalizations and 61 deaths from H1N1 flu, the majority occurring in the previous four weeks.

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August and September

In mid-August, President Oscar Arias of Costa Rica contracted swine flu. At the end of the month, President Alvaro Uribe of Colombia was diagnosed. In early September, the chief of the Ecuadorean presidential security detail died from the H1N1 flu.

On Aug. 13, Mumbai, India, schools and movie theaters closed.

On Aug. 28, Chile reported that H1N1 flu had jumped to birds. Two turkey farms were place under quarantine.

In Japan, rates of flu were much higher than usual for this time of year and according to an Aug. 28 WHO report, may signal “a very early beginning to the annual influenza season.”

Widespread outbreaks also occurred in the southeastern U.S. in August, coinciding with kids returning to school. In Georgia, many schools opened on Aug. 10. Mississippi State University has reported several hundred probable cases on campus. Deaths in school-age children have been reported in Mississippi, Alabama and Tennessee.

But around the world, the illness still looks quite similar -- in terms of severity -- to what the U.S. experienced in the spring, Longini says. This is good news for the Northern Hemisphere, which is already seeing a resurgence of flu.

It means that for most people infected with the H1N1 flu virus, illness will be no worse than seasonal flu.

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