What is influenza (flu)?
Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu vaccine each year.Flu Symptoms
Flu is different from a cold. Flu usually comes on suddenly. People who have flu often feel some or all of these symptoms:
How the Flu Spreads Person to Person
- fever* or feeling feverish/chills
- sore throat
- runny or stuffy nose
- muscle or body aches
- fatigue (tiredness)
- vomiting and diarrhea is more common in children than adults
- It’s important to note that not everyone with the flu will have a fever.
People with the flu can spread it to others up to about 6 feet away. Most experts think that flu viruses spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might get the flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose, or possibly their eyes.Period of Contagiousness
- People with the flu are most contagious in the first three to four days after their illness begins.
- Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick.
- Children and some people with weakened immune systems may pass the virus for longer than 7 days.
Symptoms can begin about 2 days (but can range from 1 to 4 days) after the virus enters the body. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Some people can be infected with the flu virus but have no symptoms. During this time, those people may still spread the virus to others.Differences between flu and COVID-19:Flu
- Flu viruses can cause mild to severe illness, including common signs and symptoms listed above.
- Flu Symptoms
- COVID-19 seems to cause more serious illnesses in some people. Other signs and symptoms of COVID-19, different from flu, may include change in or loss of taste or smell.
- COVID-19 Symptoms
Both COVID-19 and flu can have varying degrees of signs and symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms that COVID-19 and flu share include:
How long symptoms appear after exposure and infection
- Fever or feeling feverish/chills
- Shortness of breath or difficulty breathing
- Fatigue (tiredness)
- Sore throat
- Runny or stuffy nose
- Muscle pain or body aches
- Some people may have vomiting and diarrhea, though this is more common in children than adults
- For both COVID-19 and flu, 1 or more days can pass between a person becoming infected and when he or she starts to experience illness symptoms.
- If a person has COVID-19, it could take them longer to develop symptoms than if they had flu.
- Typically, a person develops symptoms anywhere from 1 to 4 days after infection.
- Flu Symptoms
How long someone can spread the virusSimilarities:
- Typically, a person develops symptoms 5 days after being infected, but symptoms can appear as early as 2 days after infection or as late as 14 days after infection, and the time range can vary.
- COVID-19 Symptoms
- For both COVID-19 and flu, it’s possible to spread the virus for at least 1 day before experiencing any symptoms.
- If a person has COVID-19, they may be contagious for a longer period of time than if they had flu.
- Most people with the flu are contagious for about 1 day before they show symptoms.
- Older children and adults with flu appear to be most contagious during the initial 3-4 days of their illness but many remain contagious for about 7 days.
- Infants and people with weakened immune systems can be contagious for even longer.
How Flu Spreads
How long someone can spread the virus that causes COVID-19 is still under investigation.
It’s possible for people to spread the virus for about 2 days before experiencing signs or symptoms and remain contagious for at least 10 days after signs or symptoms first appeared. If someone is asymptomatic or their symptoms go away, it’s possible to remain contagious for at least 10 days after testing positive for COVID-19.
How COVID-19 Spreads
How it Spreads
Both COVID-19 and flu can spread from person-to-person, between people who are in close contact with one another (within about 6 feet). Both are spread mainly by droplets made when people with the illness (COVID-19 or flu) cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs.
It may be possible that a person can get infected by physical human contact (e.g. shaking hands) or by touching a surface or object that has virus on it and then touching his or her own mouth, nose, or possibly their eyes.
Both flu virus and the virus that causes COVID-19 may be spread to others by people before they begin showing symptoms, with very mild symptoms or who never developed symptoms (asymptomatic).
While COVID-19 and flu viruses are thought to spread in similar ways, COVID-19 is more contagious among certain populations and age groups than flu. Also, COVID-19 has been observed to have more superspreading events than flu. This means the virus that causes COVID-19 can quickly and easily spread to a lot of people and result in continuous spreading among people as time progresses.
How Flu Spreads How COVID-19 Spreads
People at High-Risk for Severe Illness
Both COVID-19 and flu illness can result in severe illness and complications. Those at highest risk include:
People with certain underlying medical conditions
The risk of complications for healthy children is higher for flu compared to COVID-19. However, infants and children with underlying medical conditions are at increased risk for both flu and COVID-19.
Young children are at higher risk of severe illness from flu.
People at High Risk for Flu Complications
School-aged children infected with COVID-19 are at higher risk of Multisystem Inflammatory Syndrome in Children (MIS-C), a rare but severe complication of COVID-19.
People at Increased Risk of COVID-19 Severe Illness
Both COVID-19 and flu can result in complications, including:
Acute respiratory distress syndrome (i.e. fluid in lungs)
Cardiac injury (e.g. heart attacks and stroke)
Multiple-organ failure (respiratory failure, kidney failure, shock)
Worsening of chronic medical conditions (involving the lungs, heart, nervous system or diabetes)
Inflammation of the heart, brain or muscle tissues
Secondary bacterial infections (i.e. infections that occur in people who have already been infected with flu or COVID-19)
Most people who get flu will recover in a few days to less than two weeks, but some people will develop complications, some of these complications are listed above.
Additional complications associated with COVID-19 can include:
Blood clots in the veins and arteries of the lungs, heart, legs or brain
Multisystem Inflammatory Syndrome in Children (MIS-C)
COVID-19 Emergency warning signs