| Swine flu remains a mystery|
|Health authorities are in a race to gather information about the swine influenza outbreak, before it becomes a pandemic.|
Less than a week after the first cases appeared, questions remain over how people are becoming infected, and why it has killed some and caused only mild symptoms in others.
One thing is certain, the new strain of influenza is behaving just as public health experts expect it to - unpredictably.
"It's very hard to predict exactly," says Dr Anne Schuchat of the US Centers for Disease Control and Prevention. "We all need to all be prepared for change."
During the weekend officials learned of a total of 26 cases across the United States and in Canada, with a suspected 10 cases in New Zealand. There are more than 1300 suspected cases in Mexico.
Influenza can spread quicky, and people who are infected can spread the virus before they even start showing symptoms.
One of the biggest problems with influenza is that it causes symptoms similar to other viruses and bacteria, such as fever, headache, muscle aches, dry cough and extreme tiredness.
"The syndrome that we are hearing about in Mexico is relatively non-specific," says Schuchat.
Health officials are now testing people with influenza-like symptoms for the new strain of swine flu, which is a variant of the H1N1 virus, responsible for the Spanish Flu in 1919.
More data needed
Even if a person is known to be infected with influenza, DNA testing is required to confirm whether it is this new and unusual strain of swine flu.
"It could be all over the place and they just haven't tested for it," says Mike Osterholm, a former Minnesota state health official who is now director of the US Center for Infectious Disease Research and Policy.
Despite their proximity, the cases in Mexico, Canada and the United States are among people who apparently have nothing else in common. That suggests the connection is an as-yet unseen chain of human infections.
"It is clear that this is widespread. And that is why we have let you know that we cannot contain the spread of this virus," says Schuchat.
Professor John Mackenzie of Curtin University of Technology in Perth says more information about the virus needs to be collected.
"We really do need to know more about transmissibility of the virus in Mexico, whether the virus causing the Mexican outbreak and the one in the United States are the same virus or not," says Mackenzie.