Influenza A (H1N1) – “Swine Flu”: An Overview

** Please visit (The New England Journal of Medicine) to read Digital Disease Detection: harnessing the web for public health surveillance. N Engl J Med 360;21 May 2009. A map created for the journal focusing on Influenza A (H1N1) case detection can also be viewed by visiting: **

When the US Centers for Disease Control and Prevention (CDC) identified two cases of genetically similar swine influenza A (H1N1) viruses in southern California on April 17th 2009, no one could have predicted that before the month was through the World Health Organization’s (WHO) influenza pandemic alert level would be raised from a three to a five.  A summary of the events surrounding the Influenza A (H1N1) outbreak to date is as follows:

  • Mid-March: The Mexican village of La Gloria is put under an ‘epi-alert’ as a late outbreak of influenza appears to be affecting villagers
  • April 1:  HealthMap identified a Spanish media report that a respiratory illness has sickened up to 60% of the residents of the small Mexican village of La Gloria
  • April 2-6:  HealthMap identified additional Spanish media reports of continuing respiratory illness affecting residents of the community of La Gloria, Perote Valley, Veracruz, Mexico; residents suspect that the source of illness is related to manure “lagoons” from a local pig farm (Carroll Farms); 33 Mexican health officials are sent to the area to investigate the report of 400 ill and 2 dead
  • April 13: CDC notified of a case identified as influenza A virus in California that did not match expected subtypes
  • April 14: Determination by CDC that the virus was “swine” influenza A (H1N1)
  • April 17: Mexican authorities notify Canadian officials of an outbreak of “respiratory illness”
  • April 17: The CDC determines that two cases in neighboring counties in California due to “swine” influenza A (H1N1)
  • April 21: The WHO is notified of the cases in California
  • April 22: Reports of suspected and confirmed cases in Mexico continue to rise
  • April 23: Additional US cases are confirmed in CA, and TX
  • April 24: The WHO announces that the strains of influenza A (H1N1) in the US match the cases in Mexico
  • April 25: Mexico declares national emergency, and WHO declares a public health emergency of international concern
  • April 27: Reports begin to emerge about the possible link between a pig farm near La Gloria, Mexico and the 1st reports of illness
  • April 27: WHO raises the influenza pandemic alert level from a 3 to a 4, as confirmed cases and deaths increase in Mexico, and the US confirms 40 cases in 5 states
  • April 29: Two days later the US has 91 cases in 10 states, and 1 death confirmed by the CDC; the WHO confirms cases in 9 countries (including the US and Mexico); the influenza pandemic alert level is raised a 2nd time, from a 4 to a 5, by the WHO
  • April 29: A media frenzy ensues as suspected and confirmed cases are reported around the globe
  • April 29: Egypt orders that all pigs be culled in an effort to prevent the spread of disease
  • April 29: The WHO and other public health authorities rename the “swine flu” to Influenza A (H1N1) in an effort to dispel fears surrounding the safety of pork products (well-cooked pork products are safe to eat, and there is no risk of infection from consuming pork), and to distinguish the influenza strain as one occurring in humans
  • May 3: 200 pigs found to be infected with H1N1 virus at a farm in Alberta, Canada; suspected to have caught the virus from a Canadian who had recently returned from Mexico
  • May 4: 226 cases and 1 death have been confirmed in the United States by the CDC in 30 states; WHO has confirmed 985 cases in 20 countries
  • May 7: 2099 cases of human infection of influenza A (H1N1) in 23 countries have been officially reported by WHO

* These dates are based on our current assessment of media reports, and are subject to change as we complete our retrospective analysis. 


H1N1 is a novel influenza A virus that has begun spreading from person-to-person.  Symptoms of influenza A (H1N1) are similar to that of the regular human flu and include: fever, sore throat, fatigue, aches, chills, and cough.  Some affected by the new virus have reported diarrhea and vomiting.  It is not yet known how severe H1N1 will be. While the virus seems to be now causing only mild illness, and the numbers in Mexico appear to be leveling off, the WHO warned countries to remain vigilant and to not become complacent, as the virus may re-emerge in the fall when temperatures cool. 

HealthMap began adding frequent updates on the situation surrounding the Influenza A (H1N1) outbreak on April 24th, using the social networking tool, in an effort to distribute breaking news alerts to our users as quickly as possible (   While the media alerts have slowed, HealthMap will continue to post breaking news items on the twitter feed as they become available. Lastly, additional news on the Influenza A (H1N1) situation can be viewed by visiting   


Related Articles (more available at


Travel advisory warns of severe respiratory illness in Mexico:

‘I had a headache and fever’ says boy who survived:

Human-to-swine transmission escalates mutation risk:

WHO warns against complacency:


Other Sources for Reliable Information:


WHO Pandemic Influenza Phases:

WHO Influenza A (H1N1) Information Site:

CDC Influenza A (H1N1) Information Site:


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