The 2009 flu pandemic is a global outbreak of a new strain of influenza virus, identified in April 2009, and commonly referred to as "swine flu". It is thought to be a mutation—more specifically, a reassortment—of four known strains of the influenza A virus, subtype H1N1: one endemic in (normally infecting) humans, one endemic in birds, and two endemic in pigs (swine). Experts assume the virus "most likely" emerged from pigs in Asia, and was carried to North America by infected persons. There is also evidence that the new strain had been circulating among pigs on other continents for years before infecting humans. Virtually all transmission is human to human, with cooked pork products safe to eat as the virus cannot be transmitted by eating foods.
The outbreak began in Mexico, and there is evidence that Mexico was already in the midst of an epidemic for months before the outbreak was recognized. Soon after, their government closed down most of Mexico City's public and private offices and facilities to help contain the spread. In early June, as the virus spread globally, the World Health Organization (WHO) declared the outbreak to be a pandemic, while also noting that the virus has so far been of "moderate severity." The virus has continued to spread worldwide, especially in the Southern Hemisphere which was in its winter flu season, and to many less developed countries with limited healthcare systems. In July, because the virus was spreading internationally with "unprecedented speed", WHO announced that it would no longer require countries to submit reports of cases, but would continue to monitor "unusual events", such as outbreak clusters or surges. Laboratories were also becoming overwhelmed simply testing for the flu which was putting healthcare systems under stress.
The virus typically spreads from coughs and sneezes or by touching contaminated surfaces and then touching the nose or mouth. Symptoms are similar to those of seasonal flu, and may include fever, sneezes, sore throat, coughs, headache, and muscle or joint pains. The Centers for Disease Control and Prevention (CDC) notes that most hospitalizations have been of people that also had underlying conditions such as asthma, diabetes, obesity, heart disease, or a weakened immune system. In an attempt to slow the spread of the illness, a number of countries, especially in Asia, have enforced strict quarantines of airline passengers showing flu symptoms, including passengers seated nearby any infected persons. A number of airlines have also begun pre-screening passengers before they travel.
WHO does not expect to have a full vaccine before the end of 2009, and if any is available before that time the supply will be limited, while some countries are hoping to have vaccines earlier. The CDC states that two or three vaccine injections will be required for maximum immunity from both the swine flu and seasonal flu. There is also concern that the virus could mutate later in the year and become more virulent and less susceptible to any new vaccine.
Although most cases worldwide have been mild, experts are still very concerned, partly due to the memory of the 1918 flu pandemic, which is thought to have killed between 40 million and 100 million people, and was preceded by a wave of milder cases in the spring. In addition, Dr Anne Schuchat from CDC thinks, "this is a virus that's capable of causing a spectrum of illness that includes severe complications and death". "It's very important we take this virus seriously".
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