What is the flu (Influenza)?

Influenza, commonly known as “the flu,” is a viral respiratory infection¹ that attacks the nose, throat, and lungs. The flu is contagious, and can cause mild to very severe illness.

There are four types of flu viruses²: Influenzas A, B, C, and D. The Influenza A and B viruses most commonly spread between people and cause yearly seasonal flu epidemics. Because viruses often mutate, the spread of a new, different strain of Influenza A virus can cause a pandemic (worldwide infection).

The flu can cause malaise, fever, sore throat, congestion, and muscle aches. Symptoms from the flu generally present 1–4 days after infection, and, if not accompanied by any complications, typically last 3–7 days. In those with other complications, such as those with chronic lung disease, flu symptoms can last 2 weeks or longer.

In some cases, serious cases of the flu can cause secondary infections like pneumonia. Complications from the flu can result in hospitalization or even death.

How do you get the flu?

Flu viruses can spread through person-to-person transmission³.

You can catch the virus through particles in the air left behind after someone that is infected with the flu coughs, sneezes, or talks. People who are infected with the flu can spread it to others from up to 6 feet away. Infected particles floating in the air can be inhaled or land on your nose, eyes, or mouth.

The most common way to catch the flu, however, is through direct person-to-person contact, such as kissing or shaking hands with an infected person.

Because droplets containing viruses can remain infectious for a few hours, you can pick up the flu virus from objects and surfaces. If you don’t wash your hands before touching your face or eating, these particles can be transferred to you through your eyes, nose, or mouth.

The flu virus usually remains active on stainless steel, plastic, and other hard surfaces longer than it does on fabric and soft surfaces. The period of time in which a virus remains active outside of the body can also be affected by temperature and humidity, as well as the amount of virus deposited onto the surface.

Types of influenza

There are four types of flu viruses: A, B, C, and D. Influenzas A and B are the most common viruses in humans. Influenza C infections usually cause only mild respiratory infections, while influenza D viruses mostly infect cattle and are not known to cause infections in humans.

Influenza A viruses are divided into subtypes and can be further categorized into different strains. The current strains of influenza A viruses are influenza A (H1N1) and influenza A (H3N2). The H1N1 virus from 2009, which caused the first influenza pandemic in more than 40 years, has replaced the H1N1 virus previously circulating among human populations.

The flu virus, like any other virus, is constantly mutating⁴. This means that new strains of the flu appear every year.

Antigenic drift

The flu can mutate in two different ways. The first way, called antigenic drift, happens when small changes to the flu virus’ genetic makeup occur each time the virus mutates. Over time, these small changes compound, creating strains of the flu virus that your body may no longer recognize and be able to fight off.

Antigenic shift

The second way the flu virus mutates is called antigenic shift. This occurs when there is an abrupt and significant change in a virus. Antigenic shift is caused by a new influenza. A virus emerging from animals that is so different from the same virus found in humans that we do not have immunity against it.

This type of mutation is widely remembered as having occurred in 2009 with the spread of the H1N1 virus (popularly but incorrectly called “swine flu”).

Immunity to the flu

Both of these types of viral mutations are the reason why you can get the flu more than once. The best way to prepare for new strains of the flu is by getting a flu vaccine every year⁵.

The good news is that you are unlikely to be infected by the same exact flu strain twice. In a healthy immune system, antibodies have the ability to recognize and attack infections you have had in the past. When you are infected with a familiar flu virus, your body will respond more quickly and you are less likely to experience symptoms as severely as you did the first time you got the infection.

Who can get the flu?

While anyone can get the flu, influenza infections are most common in children. Children younger than 12 months old and adults of age 65 or older are most vulnerable, and have a higher chance of developing a symptomatic flu infection.

Adults 65 years and older, however, are at high risk of developing serious secondary infections or complications related to the flu.

How common is the flu?

According to the CDC, on average, about 5–20% of people in the United States get sick from the flu every season⁶.

During the 2017–2018 flu season, 48.8 million people had symptomatic flu infections, while 960,000 people were hospitalized from flu-related complications.

While the flu is relatively common, it carries a higher risk of causing complications that require hospitalization than the common cold.

Is the flu contagious?

As previously stated, the flu is contagious and can be transmitted in a number of ways.

Generally, people with the flu are most contagious in the first 3 or 4 days after being infected.

It’s possible, however, for you to pass on the illness from 1 day before symptoms appear up to 5–7 days after getting sick. This means that you can give others the flu virus before you know you even know you have it, making it difficult to prevent the spread of the flu.

Some people (such as those 65 and older) that rarely develop symptoms from the flu can unknowingly pass the infection on to others. Additionally, young children and people with weakened immune systems (such as those with an autoimmune disorder) could potentially infect others for longer than is normal.

What are flu symptoms?

Initially, the flu might feel like the common cold. Symptoms of the flu⁷, however, are much more abrupt and intense than those of a cold.

Common symptoms of the flu include:

• Fever or feeling feverish (note that not all cases of the flu cause a fever)
• Chills and sweating
• Muscle or body aches
• Dry, barking cough
• Fatigue or weakness
• Congestion or runny nose
• Sore throat
• Headache
• Vomiting and diarrhea (occasional, as this is more common in children and is most often seen with gastroenteritis)

If you or your child has symptoms that are very severe, don’t go away, or become worse, you should contact your healthcare provider.

How long do flu symptoms last?

Typically, flu symptoms develop about 2 days after you are infected with the virus. Symptoms can last 3–7 days, though more serious complications like a secondary infection (such as pneumonia) can take much longer to recover from. Some persistent symptoms like a cough can linger for two weeks or longer.

It’s important to remember that, even if your symptoms subside fairly quickly, you can still be contagious for 5–7 days after becoming sick.

Serious warning signs and symptoms of the flu

A number of symptoms of the flu can indicate a much more serious illness.

Adults should seek medical attention if they experience any of the following while sick with the flu:

• Difficulty breathing or shortness of breath
• Persistent pain or pressure in the chest or abdomen
• Persistent dizziness or confusion, or the inability to get up
• Seizures
• Not urinating (infrequent urination may be a sign of dehydration–it’s important to stay hydrated and drink plenty of fluids, especially when sick)
• Severe muscle pain
• Severe weakness or unsteadiness
• Fever or cough that improve, but then return or become worse
• Worsened pre-existing chronic medical conditions

Warning signs may present differently in children than in adults. You should seek medical care for your child if he or she experiences these symptoms:

• Fast breathing or trouble breathing
• Blue-tinted lips or face
• Ribs pulling in while breathing
• Chest pain
• Severe muscle pain or refusal to walk
• Dehydration (no urination for 8 or more hours, dry mouth, no tears when crying)
• Not alert or responsive when awake
• Seizures
• Fever higher than 104°F (40°C) (or any fever in children younger than 12 weeks old)
• Fever or cough that improve, but then return or become worse
• Worsened pre-existing chronic medical conditions

Complications from the flu

While the flu usually subsides on its own or with the help of antiviral medications⁸, the virus can sometimes lead to potentially life-threatening complications.

The most common flu-related complications in both children and adults include:

• Pneumonia
• Bronchitis
• Asthma flare-ups
• Heart problems
• Ear infections

Pneumonia is the most serious of these complications. Pneumonia can be life-threatening for people who are elderly or suffer from chronic health conditions.

Risk factors

Some people are more likely than others to be at risk of getting the flu or developing flu-related complications.

Factors that may increase your chances of having the flu or complications from the illness include:

• Age: People who are younger than 12 months or older than 65 years old run the highest risk of getting sick and developing complications from the flu.
• Living or working conditions: Those that work in densely populated facilities, such as nursing homes, military barracks, or schools, are more likely to be infected with the flu virus.
• A compromised immune system: People with weakened immune systems are more likely to catch the flu and develop flu-related complications than those with healthy immune systems. Your immune system can be compromised by an autoimmune disorder, cancer treatment, or HIV/AIDS.
• Chronic health conditions: Chronic conditions such as asthma, diabetes, and heart disease may contribute to the development of flu complications.
• Pregnancy: Women who are pregnant (especially in their second and third trimesters) or who are up to two weeks postpartum are more likely to develop complications from the flu.
• Obesity: People with a BMI of 40 or higher are more at risk of developing flu-related complications.
• Aspirin use under age 19: People under 19 years old that regularly take aspirin (not just as needed for pain) are at risk of developing Reye’s syndrome, a disease that causes brain and liver damage, if infected with the flu virus.

Those with a high risk of developing secondary infections or flu-related complications should call a healthcare provider immediately upon noticing symptoms.