CDC/Doug Jordan, M.A.
The outer coating of this influenza virus has been cut away, revealing its contents.
U.S. Naval Historical Center
A sign warning about Spanish Influenza at the Naval Aircraft Factory, Philadelphia, Pennsylvania, on 19 October 1918.

Symptoms and Causative Agent

Influenza is a respiratory illness caused by influenza viruses. There are two main types of influenza viruses (A and B) but many different strains of each type. The diseases caused by these viruses are often collectively referred to simply as “the flu.”

Illness from influenza can range from mild to very severe depending on several factors, including the viral strain, the patient’s age, and the patient’s health. Certain groups are at higher risk for serious complications from the flu.

Symptoms of the flu tend to emerge suddenly and include fever, chills, coughing, sore throat, achiness, headaches, and fatigue. Vomiting and diarrhea may also occur, but these symptoms are more common for children than for adults.


Influenza is primarily transmitted via infected respiratory droplets – that is, by air, via coughing and sneezing. It’s important to note that some people who are infected will not experience any symptoms (this is known as an asymptomatic infection) but will still be contagious. They can infect others without ever knowing they’re infected themselves. Even patients who experience flu symptoms may be infectious as early as a day before they first feel ill, and for up to a week after.

An important note about influenza’s ability to spread is related to its frequent genetic changes. New strains of influenza viruses appear frequently, and previous infection with a different strain does not guarantee immunity against future infection. This is one reason why the antigens in the seasonal flu vaccine usually change each year—to try to protect against whichever flu strains are currently circulating. (For more information, see “Available Vaccines and Vaccination Campaigns” below.)

Treatment and Care

Generally, flu patients are encouraged to stay home and rest, both to recover and to avoid infecting others. In mild cases, treatment is limited to addressing the symptoms of the disease: over-the-counter medicines such as acetaminophen or ibuprofen may be used to reduce fever and/or relieve aches and pains, and cough medicines or drops may be used for sore throats and to reduce coughing. Drinking extra fluids may be encouraged to prevent dehydration.

For severe cases, or for individuals at high risk for complications, physicians may prescribe antiviral medication. Many circulating influenza strains have developed resistance to available antivirals, however. Vaccination remains the primary avenue for the prevention of the flu.


Pneumonia is the most commonly seen complication of influenza infection. Typically, it is caused by a secondary bacterial infection such as Haemophilus influenzae or Streptococcus pneumoniae. The flu can also lead to sinus and ear infections, worsen existing medical conditions such as chronic pulmonary diseases, or cause inflammation of the heart.

Although any flu patient can experience complications from the disease, certain groups are at a higher risk for flu complications than others: older individuals, young children, people with asthma, and pregnant women are some of those whose risk for complications is elevated. In a typical flu season, people 65 or older account for 90% of deaths from the flu. (Some pandemic influenzas behave quite differently than expected in this regard; in the 2009 H1N1 pandemic, almost 90% of deaths from H1N1 influenza were among people younger than 65).

Available Vaccines and Vaccination Campaigns

Because new strains of influenza appear frequently, the seasonal flu vaccine usually changes each year. Each season vaccine is generally designed to protect against three strains of influenza: two “A” strains, and one “B” strain. From start to finish—the selection of which three strains to target with the vaccine, to the production of the final product—the development process for the seasonal flu vaccine can take up to eight months.

Influenza surveillance centers around the world monitor the circulating influenza strains for trends year-round. Genetic data is collected and new mutations are identified. The World Health Organization is then responsible for selecting three strains most likely to genetically resemble strains circulating in the coming winter flu season. For the northern hemisphere winter, this decision is made in the February prior. In some cases, one of the strains used in the previous year’s vaccine may be chosen again, if that strain continues to circulate. From this point, the development and production of the vaccine can begin.

Four to five months after the three vaccine strains have been selected (in June or July), the three vaccine strains that have been developed are separately tested for purity and potency. Only after individual testing is completed are the three strains combined into a single seasonal vaccine.

In the case of a pandemic, an additional vaccine may be created to protect against a particularly virulent or widespread strain of influenza. The need for a 2009 H1N1 influenza vaccine became apparent after the strains for the seasonal flu vaccine had already been selected, so that a separate vaccine was created.

A quadrivalent inactivated influenza vaccine was licensed in the United States in 2012, and a quadrivalent live virus nasal spray vaccine was licensed in 2013. These formulations include two influenza B strains in addition to the A strains. This vaccines began to be available, along with trivalent vaccines, in the 2013-14 influenza season.

U.S. Vaccination Recommendations

Influenza vaccination was added to the U.S. childhood immunization schedule in 2004. It is recommended that children, adolescents, and adults receive the seasonal influenza vaccine each year after six months of age; the inactivated vaccine is recommended for those age six months and older. A live, attenuated vaccine is available for those more than two years old and under age 50. Additional details and recommendations are specified on the immunization schedule.


Centers for Disease Control and Prevention. Epidemiology and Prevention of Vaccine-Preventable Diseases. Influenza. Atkinson W, Wolfe S, Hamborsky J, McIntyre L, eds. 13th ed. Washington DC: Public Health Foundation, 2015. (909 KB). Accessed 2/5/2016.

Centers for Disease Control and Prevention. Prevention and Control of Seasonal Influenza with Vaccines. Recommendations of the Advisory Committee on Immunization Practices, 2013-24. MMWR September 20, 2013.62;RR07:1-43. Accessed 2/5/2016.

Centers for Disease Control and Prevention. Seasonal Influenza – Key Facts About Influenza (Flu) & Flu Vaccine. Accessed 2/5/2016.

Kamps BS, Hoffman C, and Preiser W. (eds.). Influenza Report 2006. Paris: Flying Publisher, 2006. Accessed 2/5/2016.

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Last update 5 Feb 2016

Assessment Questions

Influenza Influenza is mainly spread via __________.
A) infected blood
B) infected respiratory droplets
C) dirty tissues
D) the fecal-oral route
True or false? The most common severe complication from influenza is vomiting.
A) True
B) False
Influenza vaccination is recommended for most people __________.
A) twice a year
B) yearly
C) every five years
D) once in a lifetime
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