H1N1 is a common cause of influenza in humans, with some strains being endemic to pigs or birds. The 1918 Spanish flu pandemic killed millions due to a cytokine storm, with young adults more at risk. In 2009, a new H1N1 outbreak was declared a pandemic, with symptoms similar to the flu. The virus had been circulating in pigs before transmission to humans.
The H1N1 virus is a type A influenza virus and is the most common cause of influenza in humans. In 2006, for example, H1N1 types of flu caused about 50 percent of all flu cases. While most strains of the H1N1 virus are relatively harmless, there have been several cases 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 to humans, while others are endemic to pigs or birds. The latter two are often referred to as swine flu and avian or bird flu. Various strains of the H1N1 human influenza virus are known to cause a large percentage of the seasonal flu epidemics that occur in the coldest months of each year. In most cases, these seasonal outbreaks are harmless, except for the very young or very old.
The first incidence of a pandemic H1N1 influenza outbreak occurred in 1918. This type A influenza outbreak was then referred to as the Spanish flu, but not because the virus originated in Spain. Instead, the name stuck because Spain was the only country in Europe that didn’t suppress news of the thousands of men who died of the flu while fighting in World War I. Other countries felt the news would be too demoralizing.
The Spanish flu pandemic of 1918 killed between fifty and one hundred million people in about twelve months. This strain of the H1N1 virus is thought to be particularly virulent because it caused a cytokine storm. This occurs when elevated levels of cytokines generate a feedback loop that causes an almost infinite escalation of the immune response to the virus. In the case of the Spanish flu, this resulted in massive migration of immune cells into 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 better able to develop a strong immune response, which was more likely to to lead to a cytokine storm. The weaker immune systems of the very young and old provided them with some 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 influenza virus has been confirmed to be a reassortment of genes from four strains of type A influenza. The four strains include one known to be endemic to humans, as well as a of bird flu and two different strains of swine flu. The World Health Organization officially declared the H1N1 outbreak a pandemic on June 11, 2009, noting that the declaration was due to the rapid spread of the virus and not the risk of mortality.
Infection with the H1N1 flu strain causes expected flu-like symptoms such as fever, chills, headache, muscle and joint pain, and fatigue. Vomiting and diarrhea, cough 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 a higher risk of severe H1N1 virus symptoms.
The H2009N11 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 strain of the H1N1 virus had been circulating in pigs in North America and Europe for several years before its transmission to humans. The cycling of the virus between various pig populations is thought to have facilitated the extensive mutation undergone by this particular strain.
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