Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat and sometimes the lungs. It can be mild or severe, and sometimes fatal. A flu vaccine has been shown to reduce the risk of flu and its serious complications; vaccines are updated every year.
Symptoms
Flu symptoms usually come on suddenly. People with flu often have some or all of:
- fever or feeling feverish, and chills — though not everyone with flu has a fever;
- cough and sore throat;
- runny or stuffy nose;
- muscle or body aches, and headaches;
- fatigue;
- sometimes vomiting and diarrhoea, more common in children than adults.
Some infected people have no symptoms at all: a household study during the 2017–2023 flu seasons found 8 percent of people who tested positive had none.
Timing and contagiousness
- Onset. Symptoms begin about two days after infection, with a range of about one to four days.
- Most contagious in the first three days of illness.
- Before and after. Some otherwise healthy adults can infect others from one day before symptoms start to five to seven days after becoming sick; young children and people with weakened immune systems may be contagious longer. So you can spread flu before you know you are ill.
How it spreads
Most experts believe flu spreads mainly in droplets made when people with flu cough, sneeze or talk, which land in the mouths or noses of people nearby. Less often, people catch it by touching a contaminated surface and then their mouth, nose or possibly eyes.
Who is at risk
Anyone can get flu and anyone can have serious problems from it, but some people are at higher risk of serious complications:
- people 65 and older;
- people of any age with certain chronic conditions, such as asthma, diabetes or heart disease;
- people with a body mass index of 40 kg/m² or more;
- pregnant women;
- children under 5, especially under 2.
Complications include bacterial pneumonia, ear and sinus infections, and worsening of chronic conditions such as congestive heart failure, asthma or diabetes.
How many get sick
A 2018 CDC study in Clinical Infectious Diseases, comparing two methods, suggested that on average about 8 percent of the U.S. population gets sick with flu each season, ranging from 3 to 11 percent. Children were the most likely to get sick and people 65 and older the least:
| Age group | Median attack rate |
|---|---|
| 0–17 years | 9.3% |
| 18–64 years | 8.8% |
| 65 and older | 3.9% |
That makes children under 18 more than twice as likely as adults 65 and older to develop symptomatic flu — though the chance of getting sick differs from the chance of getting very sick. Because many people never see a doctor or get tested, CDC estimates infections from measured hospitalisation rates, then divides by the population to get the seasonal incidence.
Prevention, diagnosis and treatment
- Vaccination every year is recommended.
- Everyday measures also slow respiratory germs: keeping away from sick people, covering coughs and sneezes, washing hands often, and taking steps for cleaner air.
- Diagnosis. Flu is hard to tell apart from other respiratory illnesses by symptoms alone, but tests are available.
- Treatment. Antiviral drugs can treat flu.
The viruses
Seasonal epidemics are caused by influenza A and B viruses; influenza A is divided into subtypes, such as A/H1N1 and A/H3N2, by the genes for its surface proteins, and different types and subtypes circulate over a season. Influenza A viruses are also found in many animals, including ducks, chickens, pigs, horses, whales and seals, and a flu pandemic is a global outbreak of a new influenza A virus.
Sources
- Centers for Disease Control and Prevention, "About Influenza." https://www.cdc.gov/flu/about/index.html
- Rewritten in hubnx's own words.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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