Like a common cold, the flu is caused by viruses. There are four types of influenza viruses: A and B, which are most commonly associated with seasonal flu activity and epidemics; C, which is relatively rare and causes mild respiratory illness; and D, which primarily affects cattle.
Influenza A
There are many sub types of influenza A viruses, based on two proteins — hemagglutinin (H) and neuraminidase (N) — found on the surface of the viruses. Two strains of influenza A found in human beings are the H1N1 strain and the H3N2 strain.
A novel strain of influenza A (H1N1) virus, known as swine flu because it’s typically spread among pigs, led to a flu pandemic in 2009. Between April 2009 and April 2010, the CDC estimates that there were 60.8 million swine flu cases in the United States, which led to more than 274,000 hospitalizations and nearly 12,500 deaths.
The H3N2 virus usually causes more severe symptoms and can be particularly dangerous to the young and elderly. Flu seasons that have many cases of the H3N2, such as the current 2017–2018 flu season, tend to have higher rates of hospitalizations and flu-related deaths. H3N2 is particularly resistant to the flu vaccine, and it mutates more rapidly than other strains.
Influenza B
Less common than influenza A, these viruses cause similar symptoms and can lead to outbreaks or pandemics. Influenza B is not categorized by subtypes, but there are two strains of the virus: Yamagata and Victoria.
A July 2014 study published in Clinical Infectious Diseases challenged the perception that influenza B was milder than influenza A.
Influenza C
Like influenza A and B, these viruses are found in humans. But influenza C viruses are milder and do not cause epidemics. Seasonal flu vaccines, which contain strains of influenza A and B, do not protect against influenza C viruses.
Influenza D
This strain of influenza is not known to cause illness in humans. It primarily affects cattle, though researchers note that it could eventually form a new strain that poses more of a threat to humans.