The Persistent Legacy of the 1918 Influenza Virus https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2749954/

It is not generally appreciated that descendents of the H1N1 influenza A virus that caused the catastrophic and historic pandemic of 1918-1919 have persisted in humans for more than 90 years and have continued to contribute their genes to new viruses, causing new pandemics, epidemics, and epizootics

Emerging and Re-Emerging Infectious Diseases: Influenza as a Prototype of the Host-Pathogen Balancing Act https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7126645/

Infectious diseases have always afflicted mankind and always will. New infectious diseases emerge and old diseases re-emerge as microbes adapt to new hosts and new environments.

A prototypic example of the constant struggle between microbes and man is the evolutionary success of influenza viruses as they adapt to their many hosts, including humans.

Influenza is fundamentally a recurring background or matrix disease that usually re-emerges each year in a slightly different form (antigenic drift). However, it occasionally assumes a presentation as a newly emerging disease, very different from what the global society has previously experienced

Pandemics occur when a new influenza virus variant emerges to which the human population has no immunity. Influenza A viruses are most dangerous to humans because of their wide host range, their rapid mutation rate, and their capacity to cause serious disease

In 1918, the H1N1 avian influenza virus subtype emerged, and the ensuing pandemic called the Spanish Flu killed an estimated 40–50 million people worldwide. In subsequent years, humans built up a degree of immunological memory to H1N1 influenza as it circulated in the general population, causing less severe yearly influenza epidemics.

Then, in 1957, another strain to which humans had no prior experience—the H2N2 influenza virus—appeared and triggered a pandemic that resulted in the deaths of an estimated 1–2 million people globally. In 1968, the H3N2 subtype emerged, yet another strain to which humans had no prior exposure; this pandemic resulted in the deaths of approximately 700,000 individuals. Since 1968, variants of H3N2 have circulated to cause seasonal epidemics; in 1977, human H1N1 viruses reappeared, and these also continue to circulate

Opioids and Infectious Diseases: A Converging Public Health Crisis https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6941614/

A Universal Influenza Vaccine: The Strategic Plan for the National Institute of Allergy and Infectious Diseases https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6279170/

NIH research: Think globally https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5101950/

H7N9 Avian Influenza A Virus and the Perpetual Challenge of Potential Human Pandemicity https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3705455/

The Next Influenza Pandemic: Can It Be Predicted? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2504708/

Pandemic Influenza Threat and Preparedness https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3291399/