The physical space of the MSK Library is permanently closed to visitors as of Friday, May 17, 2024. Please visit this guide for more information.
The physical space of the MSK Library is permanently closed to visitors as of Friday, May 17, 2024. Please visit this guide for more information.
Influenza (also known as "the flu") is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu vaccine each year.
In the United States, flu season occurs in the fall and winter. While influenza viruses spread year-round, most of the time flu activity peaks between December and February, but activity can last as late as May.
There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as flu season) almost every winter in the United States. Influenza A viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus:
There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively).
While more than 130 influenza A subtype combinations have been identified in nature, primarily from wild birds, there are potentially many more influenza A subtype combinations given the propensity for virus “reassortment.”
Reassortment is a process by which influenza viruses swap gene segments. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information.
Current subtypes of influenza A viruses that routinely circulate in people include A(H1N1) and A(H3N2).
Influenza A subtypes can be further broken down into different genetic “clades” and “sub-clades.”
Clades and sub-clades can be alternatively called “groups” and “sub-groups,” respectively. An influenza clade or group is a further subdivision of influenza viruses (beyond subtypes or lineages) based on the similarity of their HA gene sequences.
Currently circulating influenza A(H1N1) viruses are related to the pandemic 2009 H1N1 virus that emerged in the spring of 2009 and caused a flu pandemic (CDC 2009 H1N1 Flu website). These viruses, scientifically called the “A(H1N1)pdm09 virus,” and more generally called “2009 H1N1,” have continued to circulate seasonally since then and have undergone genetic changes and changes to their antigenic properties (i.e., the properties of the virus that affect immunity).
Influenza A(H3N2) viruses also change both genetically and antigenically. Influenza A(H3N2) viruses have formed many separate, genetically different clades in recent years that continue to co-circulate.
Influenza B viruses are not divided into subtypes, but instead are further classified into two lineages:
Similar to influenza A viruses, influenza B viruses can then be further classified into specific clades and sub-clades.
Influenza B viruses generally change more slowly in terms of their genetic and antigenic properties than influenza A viruses, especially influenza A(H3N2) viruses.
Influenza surveillance data from recent years shows co-circulation of influenza B viruses from both lineages in the United States and around the world. However, the proportion of influenza B viruses from each lineage that circulate can vary by geographic location and by season.
In recent years, flu B/Yamagata viruses have circulated much less frequently in comparison to flu B/Victoria viruses globally. Due to the COVID-19 pandemic and subsequent mitigation efforts that nearly eliminated the 2021-2022 flu season, during the 2022-2023 flu season the B/Yamagata lineage has not been sequenced anywhere in the United States.
Influenza (flu) can cause mild to severe illness, and at times can lead to death. Flu is different from a cold. Flu usually comes on suddenly. People who have flu often feel some or all of these symptoms:
*It’s important to note that not everyone with flu will have a fever.
For more information visit Flu and COVID-19 Symptoms
The time from when a person is exposed and infected with flu to when symptoms begin is about 2 days, but can range from about 1 to 4 days.
You may be able to spread flu to someone else before you know you are sick, as well as while you are sick.