The current outbreak of the swine influenza has a disturbingly similar characteristic to the Spanish Flu pandemic that occurred in 1918. This is because most of the cases reported in Mexico have been relatively healthy adults. Both the very young and the elderly, who are usually the typically hardest hit by the virus, don't appear to have been notably affected. Some of the strains that are found in the U.S. are identical to some in Mexico.
The World Health Organisation, with respect to the situation in Mexico, advises that, "Because there are human cases associated with an animal influenza virus, and because of the geographical spread of multiple community outbreaks, plus the somewhat unusual age groups affected, these events are of high concern". On April 25, they formally declared the situation to be a public health emergency of international concern.
The viruses that are of concern in this current outbreak have not been previously identified in pigs or humans. WHO has stated that the viruses appear to respond to treatment with oseltamivir, but are resistant to both amantadine and imantadine.
William Schaffner, an influenza expert at the Vanderbilt University School of Medicine has told Bloomberg that the inquiry and analysis of the outbreak "has a sense of urgency about it. They are asking us who work in hospitals to go to our emergency rooms and our paediatric wards to gather specimens and start testing them".
Health officials are very concerned because it appears to be an unusual form of virus that is implicated. New influenza viruses have throughout history shown themselves to be extremely dangerous. The 'Spanish Flu' of 1918 may have killed up to as many as 50 million people globally. Lesser, but extremely deadly, outbreaks of flu also took place on a global scale in 1957 and 1968.