The flu is caused by influenza viruses, primarily influenza A and influenza B, which are responsible for the seasonal flu that circulates every year. Influenza A viruses are particularly notable because they are the only type known to cause global flu pandemics when a new strain emerges that most people have no prior immunity to.

One reason flu keeps coming back year after year is that influenza viruses change continuously. Two processes drive this:

• Antigenic drift is the gradual, ongoing mutation of the virus over time. These small changes can make the immune system less effective at recognizing strains it has encountered before, which is part of why flu immunity does not last indefinitely and why annual vaccination is recommended.
• Antigenic shift is a more dramatic, sudden change in which two different influenza strains combine to create a new virus. These larger shifts are less common but can produce strains with pandemic potential because very few people carry immunity to them.

The flu spreads primarily from person to person through respiratory droplets released when an infected person coughs, sneezes, or talks. These droplets can land in the mouths or noses of people nearby or be inhaled. In crowded indoor spaces with limited airflow, smaller particles can also linger in the air and contribute to transmission.

It is also possible to contract the flu by touching a surface or object contaminated with the virus and then touching the nose, mouth, or eyes, though this route is considered less significant than droplet transmission.

Flu symptoms tend to arrive suddenly and can feel more intense than what most people associate with a cold. The difference in onset and severity is one of the most useful distinguishing features.

Common flu symptoms include:

• Fever or feeling feverish and having chills, though not everyone with influenza runs a measurable fever
• Cough, which is usually dry at first and can become more persistent over several days
• Sore throat, which often accompanies the early stages of the illness
• Runny or stuffy nose, which is more common with the cold but does occur with flu as well
• Muscle aches and body aches, which are often pronounced with influenza and can affect the back, legs, and arms
• Headache, which can range from mild to severe
• Fatigue and exhaustion that can be significant, sometimes making it difficult to get out of bed
• Chills and sweats, often accompanying the fever phase of the illness

Some people, particularly children, also experience vomiting and diarrhea alongside respiratory symptoms. In adults, gastrointestinal symptoms are less common.

A crucial point worth knowing: not having a fever does not rule out the flu. Some people with influenza never develop a high temperature but still feel very ill and are still contagious.

This is one of the most common questions people have, and the honest answer is that you often cannot tell for certain based on symptoms alone. Testing is the only way to confirm influenza. That said, there are general patterns that can help.

• Onset is one of the most telling differences. A cold tends to develop gradually over a day or two, while flu symptoms often come on suddenly and intensely, sometimes within a matter of hours.
• Severity also differs. The fatigue, muscle aches, and fever associated with flu are typically far more pronounced than with a cold. Many people with the flu describe being unable to do much at all for several days.
• Fever is common with flu and relatively uncommon with colds.
• Body aches are a hallmark of flu and are usually mild or absent with a cold.
• Runny nose and sneezing tend to be more prominent features of a cold than of influenza.
• Complications are much more likely with influenza. A cold rarely leads to serious illness, while flu can progress to pneumonia and other significant health problems in some people.

If you are unsure whether you have the flu, especially if you are in a high-risk group or your symptoms are severe, checking in with a healthcare provider is worthwhile. Flu tests are widely available and can provide a quick answer.

For most people with uncomplicated influenza, the most intense symptoms resolve within five to seven days, though a lingering cough and fatigue can continue for two weeks or longer. The timeline varies based on a person's age, overall health, and whether any complications develop.

A few important points about recovery:

• Fever and body aches are often the most severe in the first two to three days and then gradually improve.
• Cough and fatigue tend to linger longer than other symptoms, sometimes persisting for one to two weeks even as the rest of the illness resolves.
• Contagiousness begins before symptoms appear. Most adults can spread the flu from about a day before symptoms start until approximately five to seven days after becoming sick. This means you can pass the virus to others without yet knowing you are infected.
• Children and people with weakened immune systems may be contagious for longer than the typical window.
• Returning to normal activity before fully recovering can prolong fatigue and increase the risk of complications. Rest is a meaningful part of recovery.

If symptoms worsen significantly after initially improving, or if new symptoms develop such as shortness of breath or chest pain, reaching out to a healthcare provider is important.

While anyone can develop complications from the flu, certain groups are at meaningfully higher risk and deserve special attention when it comes to prevention and care.

• Adults 65 and older are among those most vulnerable to serious flu complications, including hospitalization and death. The immune response to both infection and vaccination tends to weaken with age.
• Children younger than 5, and especially children under 2, are at increased risk of flu complications, including pneumonia and, in rare cases, dangerous neurological complications.
• Pregnant women face higher risk of severe illness from flu, which can also affect the health of the baby.
• People with chronic health conditions such as asthma, COPD, heart disease, diabetes, kidney disease, or a weakened immune system are at substantially higher risk for complications if they get the flu.
• People who are immunocompromised due to illness, certain medications, or other causes may not mount a strong immune response to the flu and are more vulnerable to severe illness.
• People who are very overweight have been identified as a group with increased flu complication risk.

Being in one of these groups does not mean getting the flu will definitely lead to serious illness, but it does mean that prevention is especially important and that prompt evaluation when symptoms develop is a wise step.

Because influenza spreads so efficiently through respiratory droplets and close contact, it can move quickly through households, schools, workplaces, and communities. Understanding how transmission happens helps clarify the most effective ways to reduce risk.

Practical steps that help reduce the spread of flu include:

• Staying home when sick is one of the most important things you can do for the people around you. Returning to work, school, or social settings while still symptomatic puts others at risk.
• Covering coughs and sneezes with a tissue or the inside of your elbow, then disposing of the tissue and washing hands immediately, reduces droplet transmission.
• Washing hands frequently with soap and water for at least 20 seconds, or using hand sanitizer when soap is not available, helps reduce surface-to-face transmission.
• Avoiding close contact with people who are sick, and keeping distance from others when you are sick yourself, limits spread.
• Improving ventilation in indoor spaces during flu season, such as opening windows when weather permits, can reduce the concentration of airborne particles.
• Cleaning and disinfecting frequently touched surfaces during flu season can reduce the likelihood of indirect transmission.

These measures are especially important in households with high-risk individuals, where a flu infection brought in from outside can have serious consequences.

Most people with the flu recover at home with rest and supportive care. However, certain symptoms and circumstances warrant prompt medical attention.

Seek care urgently if you or someone in your care experiences:

• Difficulty breathing or shortness of breath
• Persistent chest pain or pressure
• Confusion or altered mental status
• Severe or persistent vomiting that prevents keeping fluids down
• Signs of dehydration, such as not urinating, dry mouth, or dizziness
• Symptoms that improve and then return worse than before, particularly with fever and increased cough, which may suggest a secondary infection like pneumonia

In children, additional warning signs include breathing very fast, skin that appears bluish or grayish, not waking up or interacting normally, and extreme irritability.

If you are in a high-risk group, including being older, pregnant, or having a chronic health condition, reaching out to your provider early in the course of illness is a reasonable and proactive step. There are time-sensitive options that are most effective when started early, so prompt evaluation is worthwhile.
Even if your symptoms feel manageable, do not hesitate to call your provider's office if something feels off or you are unsure whether what you are experiencing falls within a typical flu course.