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Swine influenza virus (SIV) is caused by Orthomyxoviruses that are endemic to pig populations. SIV strains isolated to date have been classified either as Influenzavirus C or one of the various subtypes of the genus Influenzavirus A.

Swine flu, unlike bird flu, is able to pass from human to human contact. The 2009 swine flu outbreak is the spread of a new strain of H1N1 influenza virus that was first detected by public health agencies in March 2009. Local outbreaks of influenza-like illness were detected in three areas of Mexico, but the presence of this new strain was not discovered for a month.

Following this discovery in the parts of United States bordering Mexico, its presence was swiftly confirmed in the local outbreaks in Mexico, and in isolated cases elsewhere. By April 27, the new strain was confirmed also in Canada, Spain, and the United Kingdom and suspected in many other nations, with over 1,800 candidate cases.

The new strain is an apparent reassortment of several strains of influenza A virus subtype H1N1, including a strain endemic in humans and two strains endemic in pigs, as well as an avian influenza.

Reassortment is common in influenza viruses, due to the structure of their genome. This particular reassortment is consistent with a transmission of swine influenza from pig to human combined with the mixing of two viral infections in the same person.

In April both the World Health Organization (WHO) and the United States Centers for Disease Control and Prevention (CDC) expressed serious concerns about this novel strain, because it apparently transmits from human to human, has had a relatively high mortality rate in Mexico, and because it has the potential to become a flu pandemic.

On April 25, 2009, the WHO determined the situation to be a formal “public health emergency of international concern�, with knowledge lacking in regard to “the clinical features, epidemiology, and virology of reported cases and the appropriate responses�.

Government health agencies around the world also expressed concerns over the outbreak and are monitoring the situation closely.

Recommendations to prevent infection by the virus consist of the standard personal precautions against influenza. This includes frequent washing of hands with soap and water or with alcohol-based hand sanitizers, especially after being out in public. People should avoid touching their mouth, nose or eyes with their hands unless they've washed their hands. If people do cough, they should either cough into a tissue and throw it in the garbage immediately or, if they cough in their hand, they should wash their hands immediately.

Many countries confirmed that inbound international passengers will be screened. Typical airport health screening involves asking passengers which countries they have visited and checking whether they feel or look particularly unwell.

On April 27, the CDC recommended the use of Tamiflu and Relenza for both treatment and prevention of the new strain.

Some physicians in the US are recommending the use of masks when in public. The purpose of a face mask is to effectively cover a person’s mouth and nose so that if a person is around someone who is infected, there is a decreased likelihood of transmission.