How do people become infected with Influenza A(H1N1)?

Outbreaks in humans are now occurring from human-to-human transmission. When infected people cough or sneeze, infected droplets get on their hands, drop onto surfaces, or are dispersed into the air. Another person can breathe in contaminated air, or touch infected hands or surfaces, and be exposed. To prevent spread, people should cover their mouth and nose with a tissue when coughing, and wash their hands regularly.

What is Influenza A(H1N1)influenza?

Influenza A(H1N1) is a highly contagious acute respiratory disease of pigs, caused by one of several swine influenza A viruses. Morbidity tends to be high and mortality low (1-4%). The virus is spread among pigs by aerosols, direct and indirect contact, and asymptomatic carrier pigs. Outbreaks in pigs occur year round, with an increased incidence in the autumn and winter in temperate zones. Many countries routinely vaccinate swine populations against swine influenza.

Influenza A(H1N1) viruses are most commonly of the H1N1 subtype, but other subtypes are also circulating in pigs (e.g., H1N2, H3N1, H3N2). Pigs can also be infected with avian influenza viruses and human seasonal influenza viruses as well as Influenza A(H1N1) viruses. The H3N2 virus was thought to have been originally introduced into pigs by humans. Sometimes pigs can be infected with more than one virus type at a time, which can allow the genes from these viruses to mix. This can result in an influenza virus containing genes from a number of sources, called a "reassortant" virus. Although Influenza A(H1N1) viruses are normally species specific and only infect pigs, they do sometimes cross the species barrier to cause disease in humans.

What are the implications for human health?

Outbreaks and sporadic human infection with Influenza A(H1N1) have been occasionally reported. Generally clinical symptoms are similar to seasonal influenza but reported clinical presentation ranges broadly from asymptomatic infection to severe pneumonia resulting in death.

Since typical clinical presentation of Influenza A(H1N1) infection in humans resembles seasonal influenza and other acute upper respiratory tract infections, most of the cases have been detected by chance through seasonal influenza surveillance. Mild or asymptomatic cases may have escaped from recognition; therefore the true extent of this disease among humans is unknown.

What are the signs and symptoms of infection?

Early signs of influenza A(H1N1) are flu-like, including fever, cough, headache, muscle and joint pain, sore throat and runny nose, and sometimes vomiting or diarrhoea.

Regarding study of the first outbreak, have you received any feedback from the WHO team sent to Mexico to investigate the outbreak?

Teams are already sending epidemiological evidence but we will know more over the next few days.

Is there any confirmation of transmission between pigs and humans at this point?

No.

Is there any information on the economic impact of the outbreak so far?

No.

Why are we so worried about this pandemic possibility when thousands die every year from seasonal epidemics?

Seasonal epidemics occur every year and we are able to treat the virus with seasonal vaccines. A pandemic is a worldwide epidemic. It is a new virus and one to which the populations will have no immunity.