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Our Physician's 

What is the Flu? Science, Symptoms, and Risk Factors to Know

November 12, 2020 9 min read

Woman in a blue shirt holding a thermometer and tissues

Dr. Sandra El Hajj - MSc, N-MD, DHSc

Influenza, also known as the flu, is an infection sickening millions every year. The virus produces mild to severe symptoms, depending on the health of its host. This infection is not the same as the stomach flu that causes diarrhea and vomiting, but a respiratory infection affecting the lungs and airways. 

Easily transmittable from one person to another, the flu comes on suddenly once symptoms begin to appear. During certain months of the year—commonly known as flu season—the virus spreads more readily among people, causing a spike in absences, doctor visits, and even hospitalizations. This article will explain more about this viral infection, its impact on populations, and how you can prevent picking it up during the winter months. 

The Flu Virus 

While the flu virus has four types (A, B, C, and D), only the first two affect humans. These are seasonal viral infections caused by the human influenza virus A and the human virus influenza B. Every year during the winter months, these two viruses spread among populations causing epidemics. 

As a contagious respiratory illness, the flu is usually accompanied by mild to severe symptoms. It has a more severe outcome on particular groups like the elderly, young children, and those who have chronic health conditions (1). 

The structures of the flu strains are constantly changing, driving a need for revised vaccines each year. Such changes occur in two different ways: the antigenic drift and antigenic shift.

The antigenic drift 

The proteins on the surface of a virus are known as antigens. When small mutations occur in the genes of a virus, its antigens are also affected. Over time, these alterations make it hard for your immune system to recognize the virus from previous exposure. This explains why you might get the flu more than once during a season, despite getting vaccinated (2). 

The antigenic shift 

When the structure of the influenza virus is drastically changed, new antigens form on the surface. This alteration leads to new subtypes of the virus and is commonly the culprit of zoonotic pandemics like Covid-19 and swine flu (3).

How is the flu virus affecting the United States? 

The influenza virus has been affecting populations of the world for centuries now. In the United States, the severity of cases fluctuates every year. While flu cases were relatively low during 2018 and 2019, 2017 was the deadliest season in decades with high hospitalization rates and emergency visits. To better understand the severity of the virus, consider the following statistics from the United States. 

Every year, roughly 5-20% of Americans get the flu. Since 2010, the CDC estimates that 9-49 million infections have occurred. Among those infected, 31 million individuals visited outpatient clinics and more than 140,000 were hospitalized (4).  

Here are a few more facts that reflect the reality of the flu in the United States: 

  • In 2017-2018, the flu was responsible for the deaths of 185 young kids. Of those, 80% were unvaccinated (5). 
  • In 2017-2018, the flu hospitalized roughly 1 million Americans. Among them, 80,000 died. 
  • In 2018-2019, 58% of hospitalized flu-infected patients were aged 65 and older.  

Aside from missed school and workdays, the flu has a significant impact on the economy. Every year, around 10.4 billion dollars are spent on medical expenses caused by the virus. The CDC adds that up to 17 million workdays are lost every year because of the flu virus, costing the nation 7 billion dollars lost in sick days (6).  

People walking in a busy hospital near a window

Signs and Symptoms 

When a person first contracts the flu virus, symptoms similar to the common cold begin to appear. These include a runny nose, sneezing, and sore throat. What differs the flu from the common cold is that its symptoms appear suddenly and aggravate rapidly. Someone infected by the flu will feel much worse than a person with a cold. Also, colds rarely cause headaches or fever, while both are common in flu infections (7). 

Among the many symptoms that accompany influenza infection, expect to experience: 

  • Fever (100.4 F or greater) 
  • Achy muscles
  • Chills and sweats 
  • Headaches and eye pain 
  • Runny or stuffy nose 
  • Shortness of breath 
  • Dry and persistent cough 
  • Vomiting and diarrhea (most common among young children) 

While most individuals infected with the flu virus show mild symptoms and do not require hospitalization, others are at high risk of developing complications due to the flu. Among them are: 

  • Children younger than five years old (children younger than six months are at higher risk of developing severe complications) 
  • Native Americans 
  • Adults older than 65 years old
  • Women who are pregnant
  • Women who just delivered (within their first two weeks postpartum) 
  • Anyone with a weakened immune system 
  • Anyone who is obese with a BMI 40+
  • Individuals with asthma, heart disease, kidney problems, and liver illnesses 
  • Diabetics
  • Americans living in nursing homes or any long-term care facility

How is the Flu Diagnosed? 

At an exam, your healthcare provider will need to gather your medical history in addition to your current symptoms. Several tests can indicate the flu. One of the most common is a rapid test where your healthcare provider swipes a swab inside your nose or on the back of your throat. The contents on the swab are then tested for the presence of the flu virus. This test is quick and reliable, yielding results within 20 minutes (8). 

Causes of the Flu 

The influenza virus is the cause of the flu. When an infected person coughs, sneezes, or talks, the virus spreads in the air as droplets. Inhaling these droplets can make you sick. Also, touching any object that may have been contaminated by an infected person can spread the virus. 

From day one, a person carrying the virus can spread it to others. The same person remains contagious up to five days after the onset of symptoms. High-risk people, like children and the elderly, may be infectious for a longer time. 

As mentioned previously, the influenza virus continually morphs into new types of the virus. If a strain resembles one you’ve already encountered, your body will be able to fight it off via antibodies. Having influenza antibodies can make the illness less severe or even prevent it from happening altogether.

The only two ways to acquire these antibodies is through vaccination or having the infection itself. When you have antibodies for an influenza virus you’ve had in the past, new influenza strains remain a threat. Think of it like this: New strains lead to the appearance of a new virus that will require new antibodies. 

Risk Factors of the Flu 

Certain groups are more prone to developing the flu than others. Other factors also put people at higher risk of complications. These are called risk factors. As explained earlier, being younger than five years and older than 65 years, being obese or pregnant, and having a Native American descent increases your risk of developing the disease or facing complications once infected. Other risk factors include: 

  • Living conditions: Where you live or work can put you at a higher risk of contracting the flu. For example, living in a multifamily home or a nursing home can increase your likelihood of exposure to the virus. Working in a healthcare setting or a school also increases your risk.
  • Having aweak immune system: When your immune system is weak, your body will struggle to defend itself against acquired illnesses, including the flu virus. Having AIDS, cancer, or being an organ transplant patient can cause complications when contracting the flu. Long-term steroid users are also at a heightened risk. 
  • Using aspirin: Those younger than 19 years of age receiving long-term aspirin therapy have an increased risk of developing Reye’s syndrome if infected by the influenza virus. This syndrome causes the swelling of the brain and liver (9). 
Adult holding a baby's hand at a hospital

Possible complications of the Flu 

The influenza virus can lead to a host of complications. While it’s not always a severe illness, many do experience discomfort during their infection. It usually takes up to two weeks for the virus symptoms to go away. However, those who develop complications will require more intensive treatment and longer recovery windows. 


When the influenza virus enters the lungs, pneumonia is a complication of great concern. It occurs when the influenza virus leads to a bacterial infection, leading to chills, fever, chest pains, and sweating.

These symptoms are usually accompanied by green, bloody phlegm; a faster pulse; and bluish lips and nails. Symptoms may appear mild at first, but bacterial infections worsen over time and require antibiotics. 

Pneumonia can last up to two weeks or longer. Fortunately, a vaccine exists that can protect young children and the elderly from such an infection. 


Bronchitis is inflammation of the bronchial tubes, which are the air tunnels bringing air out and into the lungs. Bronchitis is either acute or chronic and leads to patients coughing up thick mucus. 

Some flu complications can lead to acute bronchitis that does not necessarily need any specific treatment. Symptoms include fatigue, shortness of breath, fever, and chills as well as body aches. The condition can last up to a week with a nagging cough lingering up to a month (10). 

Asthma flare-ups 

Asthmatic people may have severe symptoms and asthma attacks when contracting an upper respiratory infection like the flu. In such cases, this can lead to wheezing and difficulty breathing. 

Heart problems 

If you have heart disease, contracting the flu can worsen your condition. Complications can include swelling of the heart muscles (also known as myocarditis) and inflammation of the heart’s protective membrane (known as pericarditis). 

In some instances, complications can produce a higher risk of having a heart attack, worsening of the existing cardiovascular condition, or even death. The flu can add more stress to the heart, leading to higher body temperature, a faster heart rate, and plaque buildup in blood vessels (11). 

Ear infections 

Among the mild complications of the flu virus are ear infections, also known as otitis media. These lead to the inflammation and swelling of the middle ear that can have symptoms like chills, fever, hearing loss, ear drainage, vomiting, and mood swings. If the pain is severe and symptoms last more than a day, a doctor needs to examine the patient to provide the best treatment possible. 


The influenza virus can lead to sinusitis, causing postnasal drip, nasal congestion, sore throat, pain in the sinuses, coughing, and a reduced sense of taste and smell. 

Sinuses are hollow air spaces found in the bones surrounding the nose. They are in charge of mucus production that drains into the nose. When the nose is blocked, sinuses are blocked and inflammation can occur. 

The flu cause sinus inflammation, referred to as acute sinusitis, lasting up to four weeks. Often treated with over the counter drugs like saline sprays and decongestants, some doctors prefer to use nasal corticosteroids to reduce the inflammation. 

Person laying on the couch covered in blankets

Treating the Flu 

The flu is a viral infection, which makes the use of antibiotics useless in managing the illness. In mild cases, antiviral drugs can help. 

For healthy individuals, managing symptoms is key. Measures for this include taking the proper medications for lowering your fever, taking cough medicine, keeping the nasal airways open by using nasal saline sprays, and maintaining a healthy diet. Getting plenty of sleep and keeping yourself hydrated are also critical for recovery.

Preventing the Flu 

The most effective preventive tool against the flu and its complications is the flu vaccine. Mayo Clinic advises everyone above the age of six months to get vaccinated yearly to reduce the severity and risk of the infection. The CDC adds that, in 2017, the flu vaccine prevented about 5.3 million illnesses, 2.6 medical visits, and more than 85,000 hospitalizations associated with the virus (12). 

As with any viral infection, you can take several measures to reduce your risk of contracting the flu (13). These include: 

  • Wash your hands with soap and water for 20 seconds or use alcohol-based hand sanitizers 
  • Avoid touching your face with unwashed hands 
  • Cover your mouth when you sneeze and cough 
  • Regularly clean surfaces to prevent contamination 
  • Avoid crowded places, especially during flu season  
  • Never visit a person who has the flu 
  • Stay at home when you’re experiencing flu-like symptoms and at least 24 hours after your fever has broken without medication

In summary

The flu is a high-impact viral infection affecting people everywhere. While most develop mild symptoms, others may experience serious complications. In both cases, stay home and take proper care of yourself under the supervision of your healthcare provider. 

With the Covid-19 virus affecting the health of populations worldwide, it is a must to keep yourself safe and protected from the flu. Getting vaccinated against the flu virus is the best step you can take to protect yourself the infection and its accompanying complications. In addition to the vaccine, make sure you’re eating a healthy diet, washing your hands frequently, and keeping your distance from those who may be ill. By taking these precautions, you’ll be well on your way to maintaining your well-being throughout the flu season.