Types of Flu

Written by Frances Gatta
Medically Reviewed by Jabeen Begum, MD on January 23, 2025
7 min read

Not all types of flu are the same. Some types of flu can make you very ill, while otherscause milder symptoms. Read on to learn about the different types of flu.

Flu, or influenza, is a contagious respiratory infection caused by various flu viruses. Symptoms include muscle aches and soreness, headache, and fever. These symptoms often go away within four to seven days with rest and over-the-counter medicines. But sometimes, they can lead to serious illness and death.

Flu vs. cold

Cold and flu share similar symptoms, such as fever, body aches, weakness, tiredness, stuffy nose, and headache, making it difficult to tell one from the other. However, these symptoms happen differently in both illnesses.

The symptoms of a cold show up slowly and are milder. You may sometimes experience aches and weakness, but headaches and fever are rare. However, you'll likely have a stuffy nose, sneezing, and sore throat. On the other hand, if you have flu, body aches, weakness, headache, and fever are common, but stuffy nose, sneezing, and sore throat only happen sometimes.

Both the flu and the common cold are caused by viruses and are contagious. However, the common cold can result from many viruses, including rhinoviruses and seasonal coronaviruses, while the flu comes only from influenza viruses. A test can confirm if you have the flu or cold.

Flu vs. COVID-19

Like the flu, COVID is also a contagious respiratory infection. But the coronavirus causes COVID, while the influenza virus causes the flu. COVID also spreads faster than the flu and remains contagious for longer periods. 

Both illnesses cause symptoms such as cough, tiredness, body aches, stuffy nose, and body aches. But loss of or changes in taste and smell happen more often with COVID than with flu. 

Unlike the flu, where symptoms appear within a day to four days of infection, COVID symptoms may take two to five days to appear after exposure. Sometimes, you may not even show symptoms until two weeks later.

COVID and flu can both lead to serious illnesses, such as pneumonia and death. But COVID can also cause other problems, such as abnormal blood clotting and long COVID, even in people with mild symptoms.

Flu viruses enter your body through the mucus membranes of your nose, eyes, or mouth. Every time you touch any of these areas, you are possibly infecting yourself with a virus. If you’re infected, symptoms of the flu can appear one to four days after exposure.

It's important to keep your hands germ-free by washing them frequently and thoroughly. Encourage family members to do the same to stay well and prevent the spread of flu.

Three types of flu viruses affect humans: types A, B, and C. Type A and B cause the annual influenza epidemics that have up to 20% of the population sniffling, aching, coughing, and running high fevers. Type C also causes flu, but its symptoms are much less severe.

The last type of flu virus, influenza D, mostly affects cattle. No cases of infection or illness have been reported in people. 

The flu is linked to about 3,000-49,000 deaths and 200,000 hospitalizations each year in the U.S. The seasonal flu vaccine was created to help prevent these epidemics.

Type A flu

Type A flu or influenza A viruses can infect animals, but it is more common for people to fall sick from this type of flu. Wild birds commonly act as the hosts for this flu virus.

Type A flu virus constantly changes and is generally responsible for large flu epidemics. The influenza A2 virus (and other variants of influenza) is spread by people who are already infected. The most common flu hot spots are surfaces that an infected person has touched and rooms they have been in recently, especially areas where they have been sneezing.

Type B flu 

Unlike type A flu viruses, type B flu is found only in humans. Although it may cause a less severe reaction than type A flu, it can still be extremely harmful on occasion. Influenza type B viruses are not classified by subtype and do not cause pandemics.

Type C flu

Influenza C viruses are also found in people but are milder than type A or B. People generally don't become very ill from type C flu viruses, and this type doesn't cause epidemics.

Different strains of the flu virus mutate over time and replace the older strains. This is why it's important to get a flu shot each year to ensure that your body develops immunity to the most recent strains of the virus.

As determined by the CDC, the viruses in a flu shot and FluMist vaccine are updated each year based on international surveillance and scientists' estimations about which flu types and strains will be most potent that year. Previously, all flu vaccines protected against three influenza viruses: one influenza A (H3N2) virus, one influenza A (H1N1) virus, and one influenza B virus. Today, FluMist and some traditional flu shots generally cover up to four strains: two influenza A viruses and two influenza B viruses.

About two weeks after getting a flu shot or FluMist, your body develops antibodies that protect against the flu viruses.

The avian influenza virus causes bird flu. Birds can be infected by influenza A viruses and their subtypes but can’t carry type B or C influenza viruses.

There are three main avian flu subtypes, including H5, H7, and H9. The subtypes H5 and H7 are the deadliest, while the H9 subtype is less dangerous.

Bird flu, particularly flu subtypes H5N1 and H7N9, can sometimes spread to humans. But it rarely ever spreads from one person to another. If you have it, it's most likely a result of coming into contact with an infected animal's fluid, such as saliva, milk, or poop, which then enters your eyes, nose, or mouth.

Bird flu symptoms include pink eye, fever, muscle aches, fatigue, sore throat, cough, diarrhea, nausea and vomiting. You're more likely to have bird flu if you work with poultry and dairy cows.

Yes, humans can get bird flu from being exposed to fluids from birds, cows, or other animals infected with the flu. Most illnesses associated with bird flu have been reported in Asian countries among people with close contact with farm birds, but cases have been reported in the U.S. after recent outbreaks in poultry and cattle populations in California, Washington, and Colorado. 

Note that people don't catch the bird flu virus by eating cooked chicken, turkey, or duck, as high temperatures kill the virus.

The CDC has created two candidate vaccines for the H5N1 bird flu and shared them with vaccine manufacturers. If there's an outbreak or pandemic, the manufacturers can make as many doses as needed for people.

It's important to know that the regular influenza vaccine does not protect against avian influenza or bird flu.

Flu is a contagious respiratory illness caused by the influenza virus. There are four types: A, B, C, and D. Only influenza A, B, and C affect humans. Flu can be mistaken for viral respiratory illnesses like the common cold and COVID because they cause similar symptoms. See a doctor if you have flu symptoms that don’t go away in a few days or get seriously ill. They can let you know if you have the flu and can recommend treatment for your case.

What are the most common types of flu?

Influenza A is the most common type of flu. 

How is the flu different from other common respiratory illnesses?

The difference between the flu and other common respiratory illnesses, like the common cold, is that the influenza viruses cause the flu. A variety of other viruses can lead to colds.

How does the flu respiratory virus spread?

The flu respiratory virus spreads from person to person. You can become infected with it if you come into contact with droplets that come out when someone with the flu coughs, sneezes, talks, or laughs.

What lifestyle adaptations can I make to avoid the flu?

Getting a flu vaccine every year, washing your hands regularly, and not touching your nose, mouth, or eyes can help you avoid the flu. 

What are the most common complications of the flu?

The most common complication of the flu is pneumonia.

How long does the flu last?

Flu can last for a few days or even up to two weeks.