The H1N1 virus is an A-type influenza virus and is the most common cause of flu in humans. In 2006, for example, H1N1 flu types caused around 50% of all cases of influenza. While most strains of the H1N1 virus are relatively harmless, there have been several instances where outbreaks of a more virulent strain of H1N1 have occurred. Such outbreaks have been noted most recently in 1918 and 2009.
Some strains of the H1N1 virus are endemic in humans, while others are endemic in pigs or birds. The latter two are often referred to as swine flu and avian or bird flu. Various strains of the human H1N1 flu virus are known to cause a large percentage of the seasonal flu epidemics that occur in the colder months of each year. In most cases, these seasonal epidemics are not dangerous, except to the very young or very old.
The first incidence of an outbreak of H1N1 pandemic flu occurred in 1918. This A-type influenza outbreak was then referred to as the Spanish flu, but this was not because the virus was known to originate in Spain. Instead, the name stuck because Spain was the only country in Europe which did not suppress news reports of the thousands of men who died from the flu while fighting in World War I. Other countries felt that the news would be too demoralizing.
The 1918 Spanish flu pandemic killed between fifty and one hundred million people over approximately twelve months. It is thought that this strain of the H1N1 virus was particularly virulent because it caused a cytokine storm. This occurs when elevated levels of cytokines generate a feedback loop which causes the immune response to the virus to escalate almost infinitely. In the case of the Spanish flu this resulted in massive migration of immune cells to the lungs, leading to extensive lung damage that was typically fatal.
Young adults were more at risk of dying than children or the elderly during the 1918 Spanish flu pandemic. This is thought to be the case because young adults were most capable of mounting a strong immune response, which was more likely to lead to a cytokine storm. The weaker immune systems of the very young and the very old gave them a certain degree of protection.
In 2009, a new outbreak of the H1N1 virus was reported. Initially referred to as swine flu, this new strain of the H1N1 flu virus was confirmed to be a reassortment of genes from four A-type influenza strains. The four strains include one known to be endemic in humans, as well as one strain of avian flu, and two different strains of swine flu. The World Health Organization officially declared the H1N1 outbreak as a pandemic on June 11, 2009, noting that the declaration was due to the rapid spread of the virus and not to the mortality risk.
Infection with the H1N1 flu strain causes expected flu symptoms such as fever, chills, headache, muscle and joint pain, and fatigue. Vomiting and diarrhea, coughing, and runny nose have also been reported. Young children, the elderly, pregnant women, and people with medical conditions such as asthma, heart disease, and diabetes may have an increased risk of serious H1N1 virus symptoms.
The 2009 H1N1 pandemic originated in Mexico, where it is believed that the virus may have circulated among the population for several months before spreading to other countries. Research indicates that this particular H1N1 virus strain had been circulating amongst pigs in North America and Europe for several years prior to its transmission to humans. It is thought that cycling of the virus between various pig populations facilitated the extensive mutation this particular strain has undergone.