Fever, cough, sore throat, and chills — these are all common symptoms of flu, or influenza. According to the millions of Americans suffer from such symptoms every flu season.

Although people often say they have “the flu,” there are actually several kinds of viruses that can cause respiratory illness. In addition to true influenza, other non-influenza viruses can also cause flu-like illnesses. Understanding the different types of flu can help you better prepare for flu season.

Flu viruses are classified into three groups: A, B and C. Influenza A and B are the most common types of viruses. These strains cause the annual seasonal flu. They’re also responsible for occasional pandemics linked to new strains and subtypes. Influenza A viruses have subtypes H and N based on differences in surface proteins. Unlike B viruses that only infect humans, A strains can cross between species.

Influenza C is not as common as A and B strains. It isn’t related to the seasonal flu or large scale outbreaks. Symptoms of influenza C are much more mild. The annual flu vaccine doesn’t offer protection against C viruses.

In addition to flu classifications, it’s also important to understand how new viruses form. Each type of flu evolves based on genetic changes. This explains how some viruses can start in animals and eventually end up causing illness in humans.

Many influenza A viruses start in birds and then directly make their way to humans. Sometimes the virus goes through pigs as well. Influenza A subtypes from different species can also mix together to form new and different viruses.

Influenza A and B are responsible for the miserable flu symptoms many Americans experience every fall and winter, including:

  • fever and chills
  • body aches, headache, and sore throat
  • cough and wheezing
  • excessive fatigue

Symptoms typically last for one to two weeks, unless other complications develop.

Seasonal flu viruses are highly contagious and mostly spread through small droplets in the air from infected persons. You can also contract influenza from touching infected surfaces and then touching your mouth, nose, or eyes. This is why regular hand washing is especially important during flu season.

According to the (CDC), a person is typically contagious one day before symptoms start and up to 5 to 7 days after.

Technically known as H1N1, this is a type of Influenza A virus. Known as “swine flu,” the strain was new to humans in 2009. It was dubbed swine flu because the virus was originally exclusive to pigs. Symptoms of H1N1 are the same as the seasonal flu.

The virus caused a pandemic between 2009 and 2010. That’s because at that time humans had no immunity against it. Since then, every seasonal flu shot offers protection against H1N1.

More commonly known as the “bird flu,” the avian influenza virus is an A virus that starts in birds. One example is the H5N1 subtype. This virus has caused numerous poultry deaths in the Middle East and Asia over the last decade.

Flu.gov reports that although this same strain doesn’t typically infect people, it has been responsible for at least 600 human cases since 2003. The strain is believed to be transmitted from birds and poultry to humans but not between people. They are no documented cases of this virus in the United States.

H7N9 is another form of avian flu virus. According to Flu.gov, no cases of H7N9 have been found in birds or humans in the U.S. Still, given the way flu viruses transform, it shouldn’t be ignored. It has caused severe respiratory infections and some deaths in China.

Flu pandemics are related to influenza A viruses that are newly introduced to the human population. Flu shots may offer immunity to known virus strains, but they can’t protect against unknown strains. This lag in lack of protection can leave humans vulnerable. The pandemic of the HINI swine flu that started in 2009 is a good example.

Despite the severity of pandemics, they’re a rare occurrence. Flu.gov documents a total of three pandemics of Influenza A that occurred during the 20th century. The best way to prepare and protect yourself during any flu season is to get vaccinated, and limit your travel and your exposure to large crowds. Equally important is to practice good hygiene habits, especially regular hand washing.

Scientists and health professionals both rely on the influenza classification process to name and diagnose new strains of flu. But flu viruses constantly change.

This phenomenon is referred to as an antigenic drift. New viruses form over time, which is why the annual influenza vaccine contains different strains from year to year.

No matter what types of viruses develop over time, you can help protect yourself and others by practicing good hygiene. This includes regular hand washing and covering your nose and mouth when you sneeze. Getting an annual flu shot is also very important to keeping yourself healthy.