What Causes the Flu?

Different Types of Flu

There are four different types of influenza viruses that exist.⁵ Some of these flu viruses have their own subtypes, lineages, groups, and sub-groups.⁵ The types of flu viruses break down into type A, type B, type C, and type D.⁵

Influenza A

Influenza A is the only type of flu viruses that can cause global flu epidemics when they spread.⁵ Influenza A is usually seasonal and occurs almost every winter in the United States.⁵ This type has two subtypes: H1N1 and H3N2.⁵ Each of those subtypes break further into different groups and sub-groups.

Influenza B

Influenza B is also a seasonal flu virus that typically occurs every winter season in the United States.⁵ Instead of subtypes, this type of virus breaks down into two lineages: B(Victoria) and B(Yamagata).⁵ Those lineages then break down further into different groups and subgroups.

Influenza C

While humans can contract Influenza C, it tends to be a mild version of the virus. Because of its mildness, it has a low likelihood to cause a flu epidemic.⁵

Influenza D

Influenza D is the fourth type of influenza virus. However, humans don’t contract this type of flu virus.⁵ It usually only affects different types of animals.⁵

It is important to check with your local and statewide healthcare providers about which flu viruses are spreading in your area because the virus is constantly changing.⁶ There are two ways flu viruses can change throughout the year:

1. Antigenic Drift

This is when the existing flu viruses mutate and change the number of surface proteins the virus has.⁶ This drift isn’t an immediate change because the viruses mutate over time as they  replicate in more people.⁶ This type of change is the main reason why people can get the flu more than once.⁶ Most of the seasonal flu shot vaccines and nasal spray vaccines target this type of flu virus change.⁶

2. Antigenic Shift

While the antigenic drift occurs over a period of time, the antigenic shift happens suddenly. This type of change occurs  when flu viruses abruptly get new proteins in their surface.⁶ Something that can cause a sudden, but major, change in the flu viruses is when a flu virus from a type of animal becomes contagious to humans.⁶ This type of change occurs rarely, and there have only been four pandemics in the past century due to this type of change in the flu virus.⁶

You can check with your local healthcare provider or the Centers for Disease Control and Prevention for any updates on changes to existing flu viruses, what types of changes are occurring, and the different flu vaccines that are being offered to offset those changes.

High-Risk Groups

There are some groups of people who are at higher risks of contracting a flu virus during the year. Some of these groups are:⁷

• Adults that are at 65 years of age and older⁷
• Children that are younger than 2 years old⁷
• People who have blood disorders (such as sickle cell disease)
• People who have heart diseases (such as congenital heart disease, congestive heart failure and coronary artery disease)
• People with chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis)
• People who have kidney diseases
• People who are obese with a body mass index [BMI] of 40 or higher.
• People with a weakened immune system from diseases (such as HIV or AIDS, or some cancers such as leukemia) or medications (such as chemotherapy or radiation)
• People who have had a stroke
• Pregnant women up to 2 weeks after the end of pregnancy