H1N1 Influenza

H1N1 influenza, a subtype of influenza A virus, is an infectious viral illness that causes both upper and, in some cases, lower respiratory tract infections in its host. H1N1 influenza infections can cause symptoms such as rhinorrhea, cough, decreased appetite, fever, rigors, myalgia, headache, and, possibly, lower respiratory tract disease and gastrointestinal disease. Although other influenza strains exist, influenza A and B viruses predominantly impact human health.

Three subtypes of swine influenza circulate globally—H3N2, H1N2, and H1N1. The H1N1 influenza gained worldwide attention as "swine flu" during the 2009 pandemic after swine influenza viruses were reassorted with preexisting H1N1 strains. Swine flu emerged from the recombination of various prior swine, avian, and human influenza strains, causing a global pandemic affecting millions of people and impacting industries, including food and tourism.

H1N1 influenza leads to a respiratory disease that can infect pigs' respiratory tract. Humans susceptible to swine influenza are typically exposed through close association with infected pigs, a condition known as zoonotic "swine flu." Swine influenza viruses can potentially infect humans if the antigenic characteristics of the virus change through the reassortment of different influenza strains. This process can enhance replication and transmission, facilitating efficient transfer to human hosts. Such reassortments have led to pandemics, as seen in 1918 and 2009, when the virus acquired efficient person-to-person transmission capabilities.

In 1918, the H1N1 influenza virus, commonly known as the Spanish flu, sparked a devastating pandemic that infected roughly 500 million individuals worldwide and led to the deaths of an estimated 50 to 100 million people, accounting for 3% to 5% of the global population at the time. This made it one of the deadliest pandemics in human history. Similarly, the H1N1 influenza strain in 2009 was classified as a pandemic by the World Health Organization (WHO). The 2009 H1N1 virus spread through airborne droplets from human to human, possibly via fomites contaminated with the virus, and subsequently transferred to the mucosa or upper respiratory tract. Notably, similarities in symptoms of H1N1 in both humans and pigs arose, potentially due to the viral reassortment of preexisting strains. This similarity in symptoms suggested a commonality in viral pathogenesis across multiple hosts, likely facilitated by reassortment, thus enhancing efficient transmission. During the pandemic, a common misconception was that individuals could contract swine flu from consuming pig products such as bacon, ham, and other pork items. However, the virus is isolated to the respiratory system and does not involve plasma, making transmission through food unlikely. This misunderstanding resulted in substantial commercial losses in the food and tourism industries.