Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. There are four types: A, B, C, and D. Influenza A and B are responsible for seasonal epidemics in humans, with A being more prevalent and capable of causing pandemics. Influenza C typically causes mild illness, while Influenza D primarily affects animals like cattle and is not known to infect humans (Types of Influenza Viruses , 2024).

Influenza is a negative-sense single-stranded RNA viruses, members of the Orthomyxoviridae family.

Simplistic structure of Influenza virus

Influenza A

Influenza A viruses are highly diverse and can infect a wide range of species, including humans, birds, and pigs. This broad host range and their ability to undergo rapid genetic changes, known as antigenic shift and drift, enable them to cause pandemics, such as the 2009 H1N1 outbreak. Influenza A is classified into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N), with 18 H and 11 N subtypes leading to combinations like H1N1, H3N2, H5N1 etc., which currently circulate in humans (Types of Influenza Viruses , 2024).

Symptoms of Influenza A include fever, cough, sore throat, muscle aches, and fatigue, typically lasting about a week, though some may experience lingering cough or fatigue. Treatment focuses on symptom relief with rest, fluids, and over-the-counter medications. Antiviral drugs, such as oseltamivir or zanamivir, can reduce illness duration if taken within 48 hours of symptom onset. Influenza A accounts for approximately 75% of confirmed seasonal flu infections and 84.1% of positive samples during the 2017-2018 season, contributing to 86.4% of hospitalizations (Seladi-Schulman, 2024).

Influenza A vs B

Influenza B viruses primarily infect humans and are less genetically diverse, evolving more slowly through antigenic drift rather than the more dramatic antigenic shift seen in Influenza A. They are classified into two lineages: B/Yamagata and B/Victoria, with B/Yamagata not detected since March 2020 CDC. Both viruses cause similar symptoms, including fever, cough, sore throat, muscle aches, and fatigue, lasting about a week. Treatment is identical for both, relying on symptom relief and antivirals (Seladi-Schulman, 2024)

While Influenza A is generally more severe, particularly with subtypes like H3N2, Influenza B can also cause significant illness. A Canadian study noted a higher mortality risk in children aged 16 and under from Influenza B compared to Influenza A Healthline. During the 2017-2018 flu season, Influenza B accounted for 15.9% of positive samples and 13.2% of hospitalizations, compared to A’s dominance (Seladi-Schulman, 2024).

Seasonal flu vaccines are designed to protect against the most common strains of both viruses, typically including two Influenza A subtypes (H1N1 and H3N2) and one or two Influenza B lineages. The vaccine’s effectiveness can vary due to viral mutations, but it remains the best preventive measure (Johnson, 2025).

Influenza C

Influenza C causes mild respiratory illness, primarily in children, and is not associated with epidemics. It has seven RNA segments, unlike the eight in A and B, and is less studied due to its lower public health impact. It is not included in seasonal flu vaccines Wikipedia. Seropositivity for Influenza C is high by age 7–10, suggesting most people are exposed during childhood (Sederdahl & Williams, 2020).

 

Naming conventions

Influenza virus follows a naming convention, shown below, that is internationally accepted (Kit, 1980).

This figure shows the naming convention for human-origin viruses

• Virus type - antigenic type (A, B, C, D)
• The host of origin (swine, equine, chicken, etc.). Note: no host of origin designation is given for human-origin viruses (image above)
• avian influenza A(H1N1), A/duck/Alberta/35/76
• Place of origin – place where the virus was first isolated or found
• Strain number
• Year isolated – year when the virus was isolated
• Hemagglutinin and neuraminidase antigen descriptions for influenza A viruses are enclosed in parenthesis (influenza A(H1N1) virus, influenza A(H3N2) virus etc.).

Conclusion

Influenza A and B are the primary drivers of seasonal flu, with A posing a greater threat due to its pandemic potential and broader host range. Influenza B, while less severe, can still cause significant illness, especially in children. Influenza C plays a minor role, causing mild symptoms. Understanding these differences informs vaccination strategies, treatment approaches, and public health planning to mitigate the impact of seasonal flu and potential pandemics.

References

Influenza virus . (n.d.). CUSABIO. https://www.cusabio.com/infectious-diseases/influenza-virus.html

Johnson, S. (2025, May 16). Influenza A vs. B: What to know. https://www.medicalnewstoday.com/articles/327397#prevention

Kit, L. S. (1980). A revision of the system of nomenclature for influenza viruses: a WHO memorandum. PubMed, 58(4), 585–591. https://pubmed.ncbi.nlm.nih.gov/6969132

Sederdahl, B. K., & Williams, J. V. (2020). Epidemiology and clinical characteristics of influenza C virus. Viruses, 12(1), 89. https://doi.org/10.3390/v12010089

Seladi-Schulman, J., PhD. (2024, July 23). How are influenza A and B different? Healthline. https://www.healthline.com/health/cold-flu/influenza-a-vs-b

Types of influenza viruses . (2024, September 18). Influenza (Flu). https://www.cdc.gov/flu/about/viruses-types.html