Highly pathogenic H5 avian influenza

The predominant natural reservoir of influenza viruses is thought to be wild waterfowl. A 2023 study involving total RNA sequencing and transcriptome mining found that Influenzaviruses and the broader Articulavirales order likely had an aquatic origin, with fish being some of the earliest hosts, and the earliest viruses in the order evolving from Crustaceans over 600 million years ago. The subtypes of influenza A virus are estimated to have diverged 2,000 years ago. Influenza viruses A and B are estimated to have diverged from a single ancestor around 4,000 years ago, while the ancestor of influenza viruses A and B and the ancestor of influenza virus C are estimated to have diverged from a common ancestor around 8,000 years ago.

Outbreaks of influenza-like disease can be found throughout recorded history. The first probable record is by Hippocrates in 412 BCE. The historian Fujikawa listed 46 epidemics of flu-like illness in Japan between 862 and 1868. In Europe and the Americas, a number of epidemics were recorded through the Middle Ages and up to the end of the 19th century.In 1918–1919 came the first flu pandemic of the 20th century, known generally as the "Spanish flu", which caused an estimated 20 to 50 million deaths worldwide. It is now known that this was caused by an immunologically novel H1N1 subtype of influenza A. …