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Kinds of Flu

Gannett Health Services
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Cornell University
Ithaca, NY 14853-3101

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Not all influenza (flu) viruses are the same. Some can make you very ill while others cause milder symptoms. People often use the term "flu" to describe a variety of mild illnesses, such as a cold or a stomach virus that has symptoms like the flu. But real flu is different. Flu hits hard and fast; its symptoms are usually worse and last longer than a cold.

General symptoms

There are several different kinds of flu virus, and even the same flu virus can affect different people in different ways. However, symptoms usually include the following:

  • Fever (temperature of greater than 100° F)*
  • Cough
  • Sore throat
  • Body aches/chills
  • Fatigue
  • Runny or stuffy nose
  • Headache
  • Occasionally diarrhea and/or vomiting

*It's important to note that not everyone with flu will have a fever.

Types of flu virus (similar but different)

ALL flu is viral and can be spread through infected droplets from breathing passages. Flu viruses are NOT “airborne.” Rather, droplets are expelled by talking, kissing, spitting, coughing, and sneezing. Droplets can spread from the infected person, either directly to other people or indirectly through hands and other surfaces before, during, and after an infected person has symptoms.

There are three main types of flu viruses: A, B, and C. However, nearly all human infections are caused by Type A or B virus. These are the typical culprits behind “regular” seasonal flu. Type C also causes flu; though, type C flu symptoms are found to be much less severe. 

  • Type A viruses infect a variety of animals as well as people. This virus is constantly changing and is most often the cause of large flu epidemics, including the 2009 H1N1 pandemic. Type A viruses are divided into subtypes, which are classified by the proteins that “sit on” the surface of the virus (i.e. H = hemagglutinin, N = neuraminidase). Type A viruses can further be broken down into strains. By nature, the virus makes small changes continually over time, but it can also make a major change and shift into a completely new subtype as it is capable of exchanging genes with other viruses when multiple viruses are infecting an individual at the same time. Having flu caused by one strain will not necessarily protect you from getting other strains. This is why it is important to get a new seasonal flu shot every year. 
  • Unlike type A flu viruses, type B is found only in humans. Type B usually causes a less severe illness than type A flu virus; but occasionally, type B flu can be extremely harmful. Influenza type B viruses are not classified by subtype, but can be broken down into different strains. Type B viruses do not cause pandemics. 
  • Type C viruses are also found in people but are milder than either type A or B. People generally do not become very ill from type C viruses. These viruses are not thought to cause epidemics and do not have subtypes.

What's not flu?

The common cold, and strep throat are both respiratory illnesses that can make you feel terrible, but neither one is caused by the influenza virus. (See "at a glance" information regarding these illnesses.)

"Stomach flu" is not really flu either. The medical term for it is gastroenteritis, which can be caused by many different viruses, bacteria, or even parasites. While vomiting, diarrhea, and being nauseous or "sick to your stomach" can sometimes be related to the flu – more commonly in children than adults – these problems are rarely the main symptoms of flu.