The Flu (Influenza): Signs, Symptoms, & Prevention

By Edo Paz, MD
Medically reviewed checkmarkMedically reviewed
October 10, 2019

For many of us, winter months inspire happy thoughts of holiday cheer and snowflakes. However, with 5-20% of Americans catching the flu each year, winter can also conjure memories of non-stop runny or stuffy noses, body aches, chills, sore throats, fatigue, and/or fevers. If you’re one of the millions of people who suffer from the flu each year, you know that these dreaded symptoms usually go away on their own after a week or so.

However, there are cases when the flu can develop into a more serious illness that may even require hospitalization. Therefore, it’s worth taking a closer look at the flu, including its causes, symptoms, how they differ from the common cold, and how long they typically last. Even if we can’t avoid the flu altogether, understanding how it spreads and affects us can potentially help us recognize and prevent complications.​

What Is the Flu (Influenza)?

The flu, short for influenza, is a viral infection that affects all or part of the respiratory system, i.e., our nose, throat, sinuses, and lungs. There are four strains of the flu virus. These are known as influenza A, B, C and D viruses. According to the Centers for Disease Control and Prevention (CDC), influenza A and B are the culprits that cause seasonal flu epidemics each winter. While the influenza C virus also causes respiratory illness, it’s known to be less severe. The remaining influenza D virus is of less concern because it doesn’t affect humans but, rather, causes illness in cattle.

Chat with a doctor and get flu treatment for just $35
Get started

What Causes the Flu (Influenza)?

The flu is caused by different viruses and is highly contagious. Since flu viruses travel through the air in droplets, they typically spread when an infected person talks, sneezes or coughs. An unsuspecting person (you!) can inhale these germ-infected droplets directly into your lungs. Keep in mind that people with the flu can spread the virus to others up to around 6 feet away.

Another way to catch the flu is by touching an infected person – either by shaking hands or wiping a sick child’s nose – and then immediately touching your face. The same goes for touching an object, say a phone, toy or utensil, that a flu-ridden person has previously touched and therefore contaminated, and then touching your face. The flu virus enters the body via the mucus membranes in our nose, mouth or eyes. So, every time your unwashed hand touches one of these areas, you run the risk of infecting yourself with a flu virus. This is why good hand hygiene is important, especially during flu season.

Another reason why the flu spreads so easily is because people with the flu virus may be contagious before they even know that they’re sick. In fact, most adults are already contagious 1–2 days before any symptoms start appearing. They remain contagious up to 7 days after their symptoms begin. Meanwhile, adults with weakened immune systems and children may be contagious for longer.

There’s yet another reason why the flu has led to between 9-49 million illnesses each year in the US. Remember the four types of flu viruses we mentioned? Each one is constantly changing, with new strains regularly surfacing. If you’ve had the flu in the past, or if you have gotten vaccinated, your body has made antibodies to fight the specific strain you were exposed to. This means that if you encounter a flu virus that’s similar, the antibodies you have formed may prevent you from catching it again or, at the very least, may lessen its severity. However, these helpful flu antibodies don’t protect against new strains of flu.

Common Flu Symptoms

Most of us have been bed-ridden with the flu at some point in our lives. While fatigue and fever are usually the first signs, not everyone with the flu will exhibit the same symptoms. The following are the most common flu symptoms:

Typically, these symptoms last for about a week. However, fatigue and/or coughing can linger for several weeks. It’s worth noting that gastrointestinal symptoms are usually more common in children.

It’s easy to confuse flu symptoms with those of a bad cold, given their similarities. But there are several indicators that can help differentiate the two. Unlike the common cold, whose symptoms tend to appear gradually, flu symptoms can come on suddenly. For example, a person who’s come down with the flu may go from feeling fine to having a fever and other symptoms within just a few hours. Plus, high fevers and body aches tend to occur more with the flu than with a cold. Overall, having the flu usually feels worse than having a cold.

Typical Stages of the Flu

As we’ve established, for most people who catch the flu, the misery usually lasts around 7 days. However, each person is different and will therefore be affected by the flu in different ways. For instance, for some people, their symptoms get better only to get worse again, which may be a sign of a different infection. Certain symptoms (like coughing or fever, for example) can also get worse at certain times of the day or night. Below is a general timeline of the stages to expect for those people who don’t develop flu complications:

Day 0: You feel absolutely fine but are already infected with the virus and are therefore already contagious and (unknowingly) spreading the virus.

Day 1: You either wake up with a fever and feel exhausted, or these (and/or other) early symptoms hit you suddenly during the day or evening.

Days 2-4: Your flu symptoms peak.

By Day 5: You may start feeling better. However, just because you feel good enough to return to work or school doesn’t mean you should. It’s recommended to stay at home until a full 24 hours have passed once your fever breaks. If the fever only goes away with anti-fever medication, stay at home.

By Day 7: Most people feel significantly better by day 7. However, for those people who are more vulnerable to the effects of the flu, either because of age or other health issues, symptoms may last longer. Also, some people may have low energy for up to 2 weeks or experience a post-viral cough for up to 8 weeks.

After Day 7: Lingering symptoms are one thing. But if any symptoms continue to get worse after a week, it’s best to see your doctor.

Flu Susceptibility and Complications: Who Is at Risk?

Some people have immune systems that are weaker than others, which means they are more vulnerable to viruses. The following people not only have an increased risk of developing the flu but may also be more susceptible to developing serious complications from it:

  • Infants and children under age 5
  • Adults older than age 65
  • People who live in nursing homes
  • Pregnant women
  • People with weakened immune systems
  • People who have chronic illnesses (like asthma, heart disease, diabetes, kidney disease, liver disease, etc)
  • People who are very obese (BMI > 40)

Flu Complications

As previously mentioned, the flu virus can lead to serious complications for those considered “high risk,” as listed above. The following flu complications can include:

These serious complications, which can take weeks or even months to recover from, most likely won’t go away without medical treatment. They can even lead to hospitalization. In fact, according to the CDC, about 200,000 people in the US need to go to the hospital each year as a result of such flu complications.

It’s also very important to note that teens and children under the age of 19 who are recovering from flu-like symptoms, (like a fever) should never take aspirin. This is because there is a risk of developing Reye’s syndrome, a rare condition that causes swelling in the brain and liver.

Flu Treatment and Prevention Options

Usually, all that’s needed to rid your body of the flu is lots of quality rest, plenty of fluids, and patience for it to go away on its own. As the flu is a viral infection, antibiotics won’t help.

However, if you have developed a serious infection or are considered to be at higher risk for complications, your doctor may prescribe antiviral drugs, like Tamiflu, especially if you are evaluated within 48 hours of symptom onset. Your doctor may also recommend the following:

If you want to lessen the chance of developing flu complications, the following home remedy tips are good to follow:

  • Stay home and get plenty of rest.
  • Limit contact with other people if possible until your fever has been gone for 24 hours.
  • Stay hydrated by drinking plenty of water, juice and warm soup.
  • Avoid alcohol and smoking.
  • Eat, if possible, to gather strength.
  • If you live alone, have a relative, friend, or neighbor check in on you periodically.

While the flu shot does reduce your risk of getting the flu, it doesn’t prevent it entirely. According to research, people who get vaccinated against the flu every year tend to have less severe and shorter-lasting symptoms.

Chat with a doctor and get flu treatment for just $35
Get started

When to See A Doctor

As we have repeated throughout this article, most people who get the flu recover on their own within a week without visiting the doctor. However, if you’re at risk of flu complications, you should see your doctor once any symptoms first appear. Additionally, if you have any severe symptoms like inability to eat or difficulty breathing you should see your doctor.

How K Health Can Help

No one wants to get the flu, but if you have it, treating the symptoms can help you to feel better. K Health’s virtual diagnosis tool can help you quickly determine whether you’re suffering from the flu or the common cold, and if you need to see a doctor. Our doctors are also available to help you learn how to ease your flu symptoms, now and in the future.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Edo Paz, MD

Edo Paz is the VP of Medical at K Health. Dr. Paz has two degrees in chemistry from Harvard and earned his medical degree from Columbia University. He did his medical training in internal medicine and cardiology at New York-Presbyterian. In addition to his work at K Health, Dr. Paz is a cardiologist at White Plains Hospital, part of the Montefiore Health System.