Flu (Influenza): Causes, Symptoms, Vaccine, Treatment

What Is The Flu (Influenza)?

Influenza, commonly known as the Flu, is a highly contagious respiratory infection caused by an influenza virus.

There are four types of influenza viruses and these are Type A, Type B, Type C, and Type D.

Three out of these four Types are known to infect humans, while the fourth Type (i.e. Type D) isn’t known to infect humans.

The Flu (Influenza) mainly affects the lungs, nose, and throat.

Its symptoms can be mild to severe. This infection can be fatal in high-risk groups like people with a weakened immune system, young children, people with chronic diseases, pregnant women, or elderly persons.

The most common symptoms of the Flu (influenza) are sore throat, runny nose, high fever, headache, cough, arthralgia, myalgia, and fatigue.

This infection directly spread through the air by coughs or sneezes by the infected person; or spread indirectly through contact with the objects (contaminated by the infected person), followed by transfer of the virus to the nose, eyes, or mouth.

Its symptoms usually appear from one to four days after initial exposure to the virus and mostly, last for seven to fourteen days.

These symptoms may last for lesser days and can be less severe in people, who had a flu shot.

What Causes The Flu (Influenza)?

The Flu (Influenza) is a highly contagious respiratory infection caused by an influenza virus.

There are four genera of influenza viruses and these are:

1) Influenza virus A,

2) Influenza virus B,

3) Influenza virus C, and

4) Influenza virus D.

These viruses are negative-strand RNA viruses and belong to the family of Orthomyxoviridae.

Influenza Virus A

These viruses cause influenza in birds and some mammals.

These can cause severe illnesses both in wild birds and domestic poultry. Sometimes these viruses get transmitted from wild birds to domestic poultry and may cause a human influenza pandemic.

Friends, I will explain to you how the subtypes of these viruses are labeled, but before that, you should know what is hemagglutinin and neuraminidase.

Hemagglutinin is a homotrimeric glycoprotein found on influenza virus surface.

And neuraminidase is an enzyme that catalyzes’ the breakdown of glycosidic linkages of neuraminic acids.

This genus has several subtypes that are labeled according to the type of hemagglutinin (called its H number) and type of neuraminidase (called its N number).

The known number of H antigens is 18 (H1 to H18) and N antigens are 11 (N1 to N11).

These virus subtypes have mutated into a number of strains with their own infective profile. As some strains can be infective to one species and not to the others, while some strains can be infective to multiple species.

H1N1, H1N2, H2N2, H5N1, H3N2, H7N7, H7N9 are some of the subtypes of influenza A virus that has been confirmed in humans.

Influenza Virus B

This genus has only one species. It is known to infect seals and human beings only.

In human beings this virus can spread only from human to human.

Influenza B virus mutates 2 to 3 times slower than influenza A virus.

Its limited host range and reduced rate of antigenic change, confirm that the influenza B virus doesn’t cause pandemics. It can cause seasonal outbreaks.

It has around 500 surface projections made up of hemagglutinin and neuraminidase.

Influenza Virus C

These viruses infect humans and pigs.

Flu caused by influenza C virus is usually milder than that caused by type A or B.

These viruses have 7 RNA segments, while type A and B have 8 RNA segments.

Influenza C virus doesn’t have the ability to antigenic drift that’s why it doesn’t cause epidemics. Therefore, it isn’t considered to be an important concern for human health. 

Influenza Virus D

These viruses infect cattle and pigs, but till date, they aren’t known to infect human beings. Like type C these also have 7 RNA segments.

Influenza D virus also doesn’t have the ability to antigenic shift, thus doesn’t cause any pandemic.

How Does The Flu (Influenza) Spread?

The flu is caused by influenza viruses, and these viruses mainly affect the lungs, nose, and throat.

These viruses spread easily from one person to another. 

When a person infected with the flu coughs, talks, or sneezes, the virus spreads through the air in droplets. And you get infected with this infection if you breathe in the virus or if you touch an object or surface that has a virus on it and then touch your own eyes, nose, or mouth.

The infected person may be able to infect other people from 1 day before their symptoms develop and up to 5 to 7 days after they become sick.

People with a weakened immune system or young children might be able to spread the infection for an even longer time, particularly if they still have symptoms.

What Are The Symptoms Of The Flu (Influenza)?

Friends, symptoms of the Flu (influenza) usually appear one to four days after initial exposure to the virus. The first symptoms are mostly chills and body aches, with fever ranging from 100 to 104 °F.

But, Friends, here one important point to be noted is approximately one-third of the patients are asymptomatic.

Some of the common symptoms of the Flu (influenza) are:


The most common symptom of the flu is an increase in body temperature of the patient i.e. fever.

The fevers associated with the flu usually range from 100°F to 104 °F.

Young children suffering from the flu may have higher fevers than the adults.

Signs like chills or sweats are also present with fever.  Mostly the fever subsides within 3 to 4 days.


A persistent dry cough is very common in the flu (influenza). A productive cough although rare can be present in the early stages of the flu (influenza).

Sometimes you may also experience chest tightness or shortness of breath. Cough associated with the flu mostly subside in 1 week but for people having underlying lung illness and in older individuals cough may last for 2 weeks or longer.   

Persons with underlying lung illness should call their doctor to prevent further complications.

Nasal Congestion

The flu virus irritates the nasal cavity and causes nasal passages to become inflamed, resulting in nasal congestion and rhinorrhea (runny nose).

This uncomfortable stuffed-up sensation (also called stuffy nose) makes it difficult to breathe.

Watery Eyes

White blood cells while fighting the infection produces substances that inflame the tear duct (which carries tears from the eye into the nasal cavity), resulting in clogging of the duct.

This results in watery eyes, tearing and a sensation of discomfort.


A severe headache may be your first symptom of the flu.

Headache in flu can be the result of infection-fighting molecules called cytokines, which are released by your immune system. These molecules can cause inflammation while fighting infection, which causes headaches in some persons.

The pressure due to inflammation of the sinus cavities to accommodate increased mucus is another cause of the headache in the flu.

Sometimes, sound and light sensitivity may also be present with headache.


Sudden and excessive fatigue is one of the symptoms of flu (influenza). This extreme fatigue can interfere with your routine normal activities.

So, it’s important to allow your body to rest which will help you fight the infection.

Muscle Pains

Muscle pains are the common flu (influenza) symptoms.

Flu-related muscle pains can manifest anywhere in the body but are most common in your back, legs, arms, and neck.  

Sore Throat

Sore throat can be present in the flu with or without cough.

Your throat may feel scratchy in the early stage of the infection. There will be a weird feeling when you swallow drinks or food.

Gastrointestinal Problems

Some strains of the influenza virus can cause gastrointestinal problems like nausea, stomach pain, diarrhea, or vomiting.


This infection can cause inflammation of the Eustachian tube (a tube links the middle ear to the throat) and the eardrum resulting in earache. This pain is sharp or dull and usually goes away on its own.

But sometimes a secondary infection can also occur in the ear, and this pain is more sharp and intense. In this case, you should see your doctor.

Flu (Influenza) Symptoms In Adults When They Should Seek Medical Help

I have explained to you the common symptoms of the flu (influenza), now I will explain to you what are the symptoms in adults when they should seek medical help.

Flu (influenza) symptoms in adults when they should seek medical help:

– Severe pain or pressure in the chest

– Confusion

– Dizziness

– Breathing difficulties

– Seizures

– Persistent vomiting

– Severe headache

– A fever or cough that was improving and then suddenly worsens

– Appearance of signs of dehydration, for e.g.  Not urinating

– Worsening of chronic medical conditions

Flu (Influenza) Symptoms In Children When They Should Seek Medical Help

Symptoms in children when they should seek medical help:

– Lethargy

– Rapid breathing

– Breathing difficulty

– Fever above 104°F

– Bluish face or lips

– Fever with a rash

– Extreme irritability

– A fever or cough that was improving and then suddenly worsens

– Appearance of signs of dehydration, for e.g. no tears when crying, not urinating for 8 hours, etc.

– Not drinking sufficient liquids

– Worsening of chronic medical conditions

Flu (Influenza) Symptoms In Babies

Flu infection in babies can be dangerous. If babies get infected with this infection then their parents should seek medical help.

Its symptoms in babies are:

-Fever of 100°F or above

– Diarrhea or vomiting

– Stuffy or runny nose

– Very tired or sleepy

– Sore throat

– Cough

The baby needs emergency medical attention if they have these symptoms:

– A fever or cough that was improving and then suddenly worsens

– Bluish color of face or lips

– Seizures

– Rapid breathing

– Fever with a rash

– Fever above 104°F

– Pulling ribs in with each breath

– Appearance of signs of dehydration, for e.g. no tears when crying, not urinating for 8 hours, etc.

– Severe and persistent vomiting

– Being so fussy that they don’t want anyone to hold them

What Are The Flu (Influenza) Complications?

Most people, who get infected with the flu, usually recover within one to two weeks’ time but some people may develop dangerous and even life-threatening complications due to the flu infection.

The most common complications of the flu are:

– Viral or bacterial pneumonia

– Ear infections

– Dehydration

– Sinus infections

– Worsening of chronic medical conditions like asthma, diabetes, etc.

– Bronchitis

– Myositis

– Acute respiratory distress syndrome

– Heart problems like myocarditis (inflammation of the heart), pericarditis, etc.

– Encephalitis

– Multi-organ failure

Ear and sinus infections are examples of moderate complications of the flu (influenza), whereas pneumonia, encephalitis, myocarditis, are examples of serious flu complications.

What Are The Flu (Influenza) Risk Factors?

These are the factors that may increase your risk of getting the flu infection or its complications are:

Weakened Immune System

Persons with weak immune systems are more likely to get infected by this infection and may also have an increased risk of developing its complications. The weakened immune system can be due to illness like cancer, HIV, etc., or due to the use of immunosuppressant medicines as in organ transplant, etc.


Children who are under 5 years of age are at increased risk of getting flu complications as their immune system isn’t fully developed.

People who are aged 65 years and older are also at increased risk of getting flu complications as their immune system weakens with age.

Chronic illnesses

People with chronic medical conditions like asthma, heart disease, diabetes, liver disease, or kidney disease, etc. are at increased risk of flu (influenza) complications.

Environmental Conditions

People who work or live in densely populated places, like schools, nursing homes, military barracks, hospitals, office buildings are at increased risk of getting flu infection.


Pregnant women (particularly in the second and third trimester) have an increased risk of flu (influenza) complications than non-pregnant women.

Premature labor or birth defects in the unborn child are the serious complications of the flu.


Obese people are more likely develop flu complications.

Aspirin Use In Children And Adolescents

Children and adolescents who have been receiving long-term aspirin therapy are at risk of developing Reye’s syndrome if they get infected with flu.

Reye’s syndrome is a rare but serious disorder that can cause liver and brain damage. This syndrome mostly affects children and adolescents, recovering from a viral infection like the flu.

Cold Vs. Flu (Influenza)

Both the cold and the flu are viral respiratory infections and have some common symptoms like:

  • Sneezing
  • Runny or stuffy nose
  • Body aches
  • Fatigue

So, it can be very difficult to tell the difference between a cold and the flu at first.

Difference between a Cold and the Flu (influenza):

1) Symptoms of the flu appear suddenly as compared to the cold.

2) Flu symptoms are more severe than the cold.

3) Flu can cause additional health problems like pneumonia, ear infections, or sepsis whereas cold rarely causes such health problems.

How Is The Flu (Influenza) Diagnosed?

Friends, here you should note some points, like:

  • People infected with the flu may have respiratory symptoms with or without fever.
  • The flu infection mostly occurs during the colder months of the year, but it can also occur in other months of the year.
  • Symptoms of the flu can be similar to other respiratory infections.
  • Approximately one-third of the patients are asymptomatic.

Thus, it is impossible to diagnose the flu based on the symptoms alone.

Therefore, to diagnose the flu (or influenza) there are several laboratory tests for the flu.

One of them is Rapid Molecular Assay. In this test, a swab is swiped inside your nose or back of your throat within 3-4 days of the onset of symptoms. This swab is tested for the influenza virus.

Some tests can give results in 15-20 minutes, but these are not as accurate as some other tests, which usually give results in one to several hours.    

How To Prevent The Flu (Influenza)?

Flu (Influenza) Vaccine

Flu (Influenza) is a very contagious disease that infects millions of people every year.

It can infect healthy individuals also and they may also spread the infection to their friends and family. In some cases, this infection can cause serious complications and can even be deadly.

Deaths related to this infection are most common in older people, but can also occur in young and healthy adults.

The best way to avoid this infection is to get its vaccination.

The Flu (Influenza) vaccine, also known as a flu shot or flu jab is recommended yearly for everyone who is aged 6 months or older. This vaccine can protect you against the infection caused by the influenza viruses and lowers the risk of serious illness, hospitalization, and death from influenza.

This vaccine is generally safe but muscle pains, feeling of tiredness, and fever can occur in some cases.

Influenza vaccine is available in following forms:

Trivalent or quadrivalent intramuscular injection

This injectable vaccine contains an inactivated form of the virus and induces protection based on the immune response to the antigens present on the virus.

Nasal spray

This form of the vaccine contains a live attenuated (i.e. weakened) form of the virus and induces protection by creating an infection in the nasal passage.

Who should get the Flu (Influenza) vaccine?

The Flu (Influenza) vaccine or flu shot is recommended annually for everyone who is aged 6 months or older. The vaccine is of much importance for people who are at high risk of developing influenza complications.

Every person should get a vaccine that is suitable for his age. There are different vaccines allowed for persons of different ages.

Like the recombinant influenza vaccine (RIV) is allowed for young adults, and high-dose inactivated vaccines are recommended for older people of 65 years or more.

Influenza vaccines are generally produced using eggs but still suggested for people having egg allergies.

Egg-free vaccines are also available and can be given safely to people having severe egg allergies.

Pregnant women can also get an influenza vaccine, irrespective of their stage of pregnancy.

Breastfeeding women can also get this vaccine to protect them and to pass on antibodies to their babies.

Who should not get the flu (influenza) vaccine?

These people should not get the vaccine:

  • Children who are younger than 6 months of age
  • People who had a severe allergy to the vaccine in the past.

Precautions To Be Taken To Reduce The Spread Of The Flu (Influenza)

The influenza vaccine is very helpful in preventing influenza infection, but it is not 100% effective. So, people must take some precautions to reduce the spread of this infection, and these precautions are:

1)  Wash your hands

This step can be the best measure to prevent the spread of this virus.

You should clean your hands with soap and water for at least 20 seconds, and if soap and water are not available (like, if you are visiting public places) you can use alcohol-based hand sanitizer.

Cleaning your hands becomes more necessary in situations like if you are coming from shopping, or after traveling in public transport, etc. Cleaning your hands frequently will destroy the virus you might have acquired from touching the surfaces used by other persons.

2)  Use of face mask

Use of face mask when you are around or near a person, who is infected can prevent you from getting infected with the flu.

3) Avoid touching your face when an infected person is near you

You should avoid touching your face (especially your nose and eyes areas) when a person infected with the flu is around you and he/she is contaminating the surfaces.

4) Keep your household surfaces clean

An infected person can contaminate doorknobs, light switches, remote controls, countertops, etc. by using them.

So, you should frequently disinfect these surfaces with a good disinfectant, when someone in your family is suffering from the flu.

5) If a family member is suffering from the flu then avoid sharing items

Friends, if someone in your family is infected with this infection, then you shouldn’t share drinking glasses, cups, or utensils with that member.

This could be very helpful in preventing the spread of this disease in children.

6) Avoid crowds

This infection spreads very easily in places where people gather like in schools, auditoriums, office buildings, and public transportation. Avoiding crowds during peak flu season can reduce your chances of getting infected with the flu.

7) Boost your immune system

Your immune system protects you from infection. When your immune system is functioning properly, it launches an attack on threats, like the influenza virus.

Maintaining a healthy lifestyle like doing exercise regularly, eating a balanced diet, reducing stress, and getting enough sleep is very important; it will make your immune system strong.

Your strong immune system will fight the flu easily if you get suffered from the infection.

When Is The Flu Season?

An annually repeating time period characterized by the occurrence of outbreaks of the flu (influenza) is called Flu season.

There is no fixed date for the flu season, but it usually occurs in cold months of the year in each Hemisphere.

The flu infection can occur throughout the year and cause few infections, but in cold weather, this infection is exponentially more active.

Since the winter season occurs at different times in the Northern and Southern Hemisphere, so there are two flu seasons every year.

Due to this reason WHO recommends two formulations of the flu vaccine each year, one for the Southern Hemisphere and another for the Northern Hemisphere.

The flu vaccine can lessen the effects of the flu season and also lowers a person’s risk of getting this infection.

What Is The Treatment Of The Flu (Influenza)?

Over-the-counter (OTC) medicines For The Flu (Influenza)

The most commonly used Over-the-counter (OTC) medicines for the flu usually contain antihistamines, pain relievers, and decongestants.

Pain Relievers    

For getting relief from symptoms like fever, headache, and muscle ache, pain relievers are used. Mostly used pain relievers are acetaminophen and ibuprofen.


These may relieve symptoms like runny nose, sneezing and watery eyes.


Decongestants are used to reduce the symptoms of a stuffy nose and are helpful in relieving congestion in the upper respiratory tract. These are usually avoided in people with high blood pressure, as they may increase blood pressure.

Cough Syrups

Cough is a common flu symptom. The cough syrups containing cough suppressants like dextromethorphan help to relieve dry cough.


This medication helps to loosen mucus and phlegm, making it easier to blow your nose or clear your throat congestion by coughing. 

You should avoid using the unnecessary medication as it can cause unwanted side effects.

Prescription Medications (Antiviral Drugs) For The flu (Influenza)

Prescribed medicines i.e. antiviral drugs for the flu prevent the virus from growing and replicating.

These drugs slow down the spread of the influenza virus within the body and thus, reduce the symptoms of the disease and prevent its complications. These medications help in faster recovery.

The commonly prescribed antiviral medications are:


Zanamivir is available as a powdered form that is inhaled through a device just like an asthma inhaler. It can be given to people who are seven years or older.

Zanamivir should be avoided in people with chronic respiratory problems like asthma.


Peramivir is given intravenously. It can be given to people who are 2 years and older.


Oseltamivir is available in liquid and capsule form and can be prescribed for young children and adults.


Baloxavir is available in the market as a single-dose pill and can be given to people who are 12 years and older.

Older antiviral drugs like amantadine are not recommended nowadays because most circulating strains of the flu virus are resistant to them.

These drugs must be started within 48 hours of the appearance of symptoms for maximum effect.

Side Effects Of These Antiviral Drugs

These antiviral drugs have side effects like lightheadedness, nausea and vomiting.

Frequently Asked Questions

What is the incubation period for the flu (Influenza)?

The incubation period for the flu means the period between the exposure to the flu virus and the appearance of its initial symptoms. Generally, it is between one to four days and varies from person to person.

The average incubation period for the flu is 2 days.

How long does the flu (Influenza) last?

The flu is a very contagious and short-term illness. Its initial symptoms usually appear within one to four days after exposure to the virus. Its symptoms generally last for five to seven days.

The symptoms may last for a shorter duration of time or maybe less severe for the people who have received the flu vaccine.

Some people may experience cough or fatigue for a two week or longer.

 If complications get developed in the flu, they may take longer time to resolve.



This article is intended for informational purposes only. Any information associated with this article should not be considered as a substitute for prescriptions suggested by local health care professionals.


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