Influenza, known as “the flu,” is a highly contagious respiratory virus. It’s most common during the fall and winter months. It typically spreads through respiratory droplets when a person who has the flu sneezes or coughs.

The family of viruses that influenza is a part of is large. You may have heard that there are different types of influenza viruses — particularly influenza A and influenza B.

There are actually four different types of influenza viruses: influenza A, B, C, and D.

Influenza A and B are the two types of influenza that cause epidemic seasonal infections nearly every year.

Influenza A can be found in many species, including humans, birds, and pigs. Due to the breadth of potential hosts and its ability to genetically change over a short amount of time, influenza A viruses are very diverse. They’re capable of causing a pandemic. This happens when a virus that’s significantly different from circulating influenza A strains emerges.

Influenza B is typically only found in humans.

Influenza C mainly occurs in humans, but has been known to also occur in dogs and pigs.

Influenza D is found mainly in cattle. According to the Centers for Disease Control and Prevention (CDC), it’s not known to infect or cause illness in humans.

Influenza A is further divided into different subtypes. These subtypes are based off of the combination of two proteins on the viral surface: hemagglutinin (H) and neuraminidase (N). There are 18 different H subtypes and 11 different N subtypes.

For example, the most common influenza A subtypes that go around seasonally in humans are H1N1 and H3N2. In 2017, H3N2 spread to dogs in Florida. In 2015, this same strain also infected dogs in an earlier outbreak in Chicago.

Influenza A viruses can be further broken down into strains.

Unlike influenza A, influenza B isn’t further divided into subtypes. But it can be broken down further into specific viral lineages and strains.

The naming of influenza virus strains is complex. It includes information such as:

  • influenza type (A, B, C, or D)
  • species of origin (if isolated in an animal)
  • geographical origin
  • strain number
  • year of isolation
  • H or N subtype for influenza A

It’s estimated that influenza A infections account for 75 percent of confirmed seasonal influenza infections overall. Influenza B infections account for the remaining 25 percent.

While most confirmed infections during flu season will be influenza A, the occurrence of influenza B infections can increase late in the flu season. This happened in the 2017 to 2018 flu season.

Both influenza A and influenza B are extremely contagious. People who get either type can spread the virus to others from up to six feet away when they cough or sneeze.

You can also contract the virus by touching a surface that has the virus on it and then touching your nose or mouth.

Treatment for an influenza infection is the same regardless of the type you’ve contracted.

Unfortunately, there’s no treatment that can kill the virus. Treatment is focused around relieving symptoms until your body clears the virus naturally.

Antiviral medications may decrease the amount of time that you’re ill, which may also reduce your symptoms. Common antiviral prescriptions include:

There is also an antiviral drug called baloxavir marboxil (Xofluza) that was approved by the U.S. Food and Drug Administration (FDA) in late 2018.

The zanamivir, oseltamivir, and peramivir medications noted above work by reducing the ability of the virus to release itself from infected cells. The newer drug, baloxavir marboxil works by reducing the ability of the virus to replicate.

These antiviral medications are most effective when started within the first 48 hours of your illness. They’re ineffective in treating illness caused by influenza C.

Over-the-counter medications can be taken to relieve nasal congestion, fever, and aches and pains.

Getting lots of rest, eating a healthy diet, and drinking plenty of fluids helps your body fight the virus as well.

An uncomplicated infection with either influenza A or influenza B can cause symptoms that last around one week. Some people may still have a cough or feel fatigued after two weeks.

Some influenza A subtypes can cause more severe disease than others. For example, in the recent past influenza A (H3N2) viruses have been associated with more hospitalizations and deaths in children and the elderly than in other age groups, according to the CDC.

In the past, it was thought that infection with influenza A was more severe than infection with influenza B. However, a 2015 study in adults with influenza A and influenza B found they both resulted in similar rates of illness and death.

Additionally, in a Canadian study looking at children 16 years old and younger, influenza B infection was associated with a higher risk for mortality than influenza A.

Influenza C is regarded as the least serious of the three types that humans can get. It typically produces a mild respiratory illness in adults. But there’s some evidence that it can cause serious respiratory illness in children under age 2.

The CDC estimates that each year, from 2010 to 2018, influenza infection resulted in between 9.3 and 49 million illnesses, 140,000 to 960,000 hospitalizations, and 12,000 to 79,000 deaths.

Data for the 2017 to 2018 influenza season indicates that 84.1 percent of positive samples were influenza A, while 15.9 percent were influenza B. Among hospitalizations, 86.4 percent were associated with influenza A, while 13.2 percent were associated with influenza B infection.

The seasonal flu vaccine is developed many months in advance of flu season. The viruses selected for the vaccine are based on research into which strains will likely be most common.

Sometimes the circulating influenza viruses can mutate from one season to the next. Since experts must select the viruses to include in the vaccine months before flu season, there may not be a good match between the vaccine and the circulating viruses.

This can lead to a decrease in the effectiveness of the vaccine. But even when this happens, the vaccine still offers some protection.

Flu vaccines can be either trivalent or quadrivalent.

A trivalent vaccine protects against three flu viruses:

  • H1N1 influenza A virus
  • H3N2 influenza A virus
  • influenza B virus

A quadrivalent vaccine protects against the same three viruses as the trivalent vaccine plus protection against an additional influenza B virus.

Influenza C virus isn’t included in influenza vaccines.

There are several different types of influenza virus: A, B, C, and D.

Influenza types A, B, and C can cause illness in humans. But types A and B cause seasonal epidemics of respiratory illness nearly every year.

Influenza A typically causes the majority of illnesses during flu season. It has the potential to lead to pandemics due to its dynamic, faster changing nature and large host range.

Both influenza A and B are extremely contagious and cause the same type of illness and symptoms. While there’s no cure for influenza virus, antiviral medications, plenty of fluids, and rest can help your body fight the infection.

Yearly vaccination can also help you prevent contracting influenza A or B.