The flu is a contagious viral infection of the respiratory system that affects the nose, throat and lungs. It is caused by influenza viruses.
Symptoms of mild flu infections can resemble those of the common cold, which is also caused by viruses; that said, flu symptoms tend to be more intense than cold symptoms and come on faster. And in severe cases, flu symptoms can be serious enough to lead to hospitalization and death.
Flu viruses are categorized into four types: A, B, C and D. Types A and B are the most harmful to humans and cause seasonal flu epidemics; in the Northern Hemisphere, flu season typically occurs from October to May, with peak activity from December to February.
Influenza C can also infect people but causes very mild illness and is not associated with epidemics, while type D affects cattle and other animals and is not known to cause illness in humans.
Flu spreads between people via viral particles in droplets expelled when an infected person coughs or sneezes, or through saliva exchanged via shared utensils or kissing.
Each year, seasonal flu sickens about 1 billion people worldwide and causes approximately 290,000 to 650,000 respiratory deaths. In a given season, the U.S. sees about 200,000 hospitalizations and 36,000 deaths from flu, although these numbers vary year to year.
With rare exceptions, seasonal vaccines are recommended for everyone 6 months and older to avoid illness and to prevent serious complications from flu. Hand-washing, avoiding contact with sick people and masking while in public can also reduce the risk of infection.
Facts about the flu
- Cause: Influenza viruses
- Early symptoms: Fever, chills, tiredness, muscle aches
- Prevention: Seasonal vaccinations
Everything you need to know about the flu
What are the symptoms of flu?
Although flu is primarily a respiratory illness, its symptoms affect the entire body.
Fever is one of the most common flu symptoms, and it commonly lasts for three to four days. Other symptoms include chills, severe headache, sore throat, cough, chest discomfort, and a stuffy or runny nose. Flu also brings muscle aches and extreme fatigue. In some instances, flu may cause vomiting or diarrhea; this occurs more often in children than in adults.
Symptoms vary from person to person, and they may be mild or very severe. In some cases, flu can lead to pneumonia, a severe lung infection, and require hospitalization; it can also raise the risk of serious secondary infections from bacteria. People are more at risk of developing serious complications from flu if they have one or more underlying health conditions, such as diabetes; asthma or chronic lung disease; heart or blood disease; and liver or kidney disorders.
Flu is more dangerous in people who are pregnant or who are age 65 or older. Children under 5 are vulnerable to severe flu, and those under 2 are at especially high risk for developing serious illness. Having a weakened immune system due to disease or certain medical treatments, such as chemotherapy, can also raise the risks of life-threatening complications.
What’s the difference between a cold and the flu?
Flu and colds are the most common respiratory infections in the United States. Both affect the tissues of the nose and throat, and both are caused by viruses and spread easily between people. However, there are some key differences between colds and the flu.
Colds are caused by approximately 200 different viruses, including rhinoviruses, adenoviruses and coronaviruses (a broad group of viruses that includes the germ behind COVID-19).
Cold symptoms usually appear within one to three days of being infected and are mild at the start. They often include a tickle in the throat, sneezing, and a runny or stuffy nose. Symptoms typically peak within a week of onset, around Day 4 to 7. Headache or fever may develop, but these symptoms are rare in colds. A cold’s average duration is eight to 10 days. Most colds go away on their own, but over-the-counter medications can help relieve symptoms and reduce pain.
Flu comes on faster than a cold. In flu, fever, chills, headaches, body aches and exhaustion are typically the first symptoms to appear. Flu infections can cause deadly complications and can worsen existing medical conditions, such as asthma, congestive heart failure or diabetes.
There are no medications to prevent or directly treat the common cold, whereas flu can be prevented through seasonal vaccines and can be treated within the first 48 hours of infection with the antiviral medication oseltamivir, better known asTamiflu.
How long does the flu last?
Flu symptoms typically appear about one to four days after exposure to the virus, but flu symptoms can rapidly become severe. If the infected person is otherwise healthy, symptoms usually last from a few days to nearly two weeks, although being vaccinated and taking Tamiflu can shorten their duration. Flu symptoms may last longer in some cases and can cause fatigue that persists even after other symptoms have ended.
For most people, a bout of flu is unpleasant and uncomfortable but not life-threatening. However, complications can develop that may prolong illness. Some complications are moderate, such as ear and sinus infections.
Other complications are more serious, such as viral or bacterial pneumonia. This inflammation of the lungs occurs when the lung sacs, known as alveoli, fill with fluid and hinder breathing. Recovery from pneumonia can take up to a month or longer. Flu can also trigger sepsis, an extreme, body-wide immune reaction. Runaway inflammation driven by sepsis can lead to tissue damage and fatal organ failure.
What’s the difference between influenza A and influenza B?
Bird flu, or avian influenza, is caused by influenza A viruses. Flu viruses that infect humans and birds are both categorized by their H and N proteins, but avian influenza viruses infect birds as their primary hosts.
All of the influenza A subtypes are found in birds, with two exceptions: subtypes H17N10 and H18N11, which occur only in bats. A handful of subtypes of avian influenza A viruses are known to occasionally infect humans: H5, H6, H7, H9 and H10.
Flu spreads between birds through contact with infected saliva, respiratory droplets or feces. Some strains of avian flu cause mild illness in birds; others are deadly. Infected birds may sneeze or cough, develop swelling in various body parts, and appear uncoordinated or lethargic. That said, some birds can carry and spread flu viruses without falling sick themselves.
In people, avian flu symptoms resemble those of seasonal flu and can range from mild to severe, sometimes turning deadly. Symptoms also may include conjunctivitis — eye irritation and redness.
There are strains of bird flu that affect cattle and may be able to sicken people who drink raw milk from infected cows.
Person-to-person transmission of bird flu has historically been reported in isolated cases involving one to two individuals, but not during the ongoing outbreak in the U.S. Avian flu does not spread easily from person to person, but there is a growing risk that bird flu could mutate to become more contagious in humans, raising the risk of a bird flu pandemic.
Glossary
- Sepsis: A life-threatening medical emergency caused by the body’s extreme response to an infection. During sepsis, the immune system ceases to fight invading pathogens and begins attacking body tissues and organs.
- Pneumonia: An infection that causes inflammation in lung tissue, filling the lungs with pus or fluid and restricting breathing. Pneumonia can occur in one lung or in both, and it may be caused by viruses, bacteria or fungi.
- Rhinovirus: A type of virus that affects surface cells in the respiratory system. Rhinoviruses are the most frequent cause of the common cold. They spread through the air and through contact with contaminated surfaces.
- Viral load: How much of a virus a person is carrying in their body. The more the virus replicates in a person, the higher their viral load will be. This typically makes the person more likely to spread the infection, as well.
Flu pictures
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Discover more about the flu
—How do people die of the flu?
—How long is the flu contagious?
—How to get better faster when you have the flu, according to science
This article is for informational purposes only and is not meant to offer medical advice.
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