Health experts are fond of saying any new disease is just a flight away from anywhere, and a report shows the new strain of H1N1 flu followed the airline route map as it spread around the globe.
The swine flu virus spread first and quickest in March and April in the United States and Canada - where 80 percent of airline passengers travelled in March and April of 2008, researchers at St. Michael's Hospital in Toronto found.
Dr. Kamram Khan of St. Michael's and colleagues used International Air Transport Association data for their study. They said travel patterns were also similar in 2007 and therefore likely to be similar in 2009.
"This work provides the world with a potent early warning system for emerging infectious diseases," Dr. Michael Gardam of the Ontario Agency for Health Protection and Promotion said in a statement.
"Our analysis showed that in March and April 2008, a total of 2,35 million passengers flew from Mexico to 1 018 cities in 164 countries," Khan and colleagues wrote in the New England Journal of Medicine and published at http://h1n1.nejm.org/.
Los Angeles had the most travellers, with 221 494 passengers arriving from Mexico in March and April of 2008. New York followed with 126 345. The first non-US city was Toronto, No. 12, with 44 854 passengers arriving from Mexico.
The World Health Organisation has confirmed 70 893 cases of the new H1N1 swine flu pandemic, with 311 deaths. However, US health officials last week said there were likely at least one million cases there alone. Iraq, Lithuania, Monaco and Nepal all confirmed their first cases on Monday.
The first cases were detected in two California children in April, but tests showed the first known infections were in Mexico. By the time they had identified a new virus, US officials said it had spread too far and too fast to try to stop it.
Other countries tried measures to slow its spread, but WHO declared a pandemic - a global epidemic - this month.
"For the first time, we can quickly integrate information about worldwide air traffic patterns with information about global infectious disease threats," Khan said.
"What this means is that cities and countries around the world can now respond to news of a threat earlier and more intelligently than ever before."
The researchers said the United States receives more than 76 million international visitors from around the world every year and the United States and Canada together generate and receive about one-sixth of the global volume of international air traffic.