Influenza: What is it? Types of Viruses, Causes, Symptoms, Complications, Diagnosis, Treatment and Prevention

Generally confused with the flu, it is a highly infectious respiratory viral disease.

The disease is caused by certain strains of the influenza virus.

When the virus is inhaled, it attacks cells in the upper respiratory tract, causing typical flu symptoms such as fatigue, fever and chills, dry cough, and body aches.

Flu victims are also susceptible to life-threatening secondary infections.

Although commonly blamed for stomach upsets and diarrhea, the influenza virus rarely causes gastrointestinal symptoms.

Such symptoms are probably due to other organisms such as rotavirus, Salmonella, Shigella, or Escherichia coli .

The flu, or influenza, which occurs predominantly in the winter months. It is easily mistaken for the common cold, which is caused by a different virus.

The flu is considerably more debilitating than the common cold. Flu outbreaks happen suddenly and the infection spreads quickly.

Flu outbreaks happen regularly. The most serious outbreaks are pandemics , affecting millions of people around the world and lasting for several months.

The flu outbreak from 1918 to 1919 serves as the prime example of an influenza pandemic, the death toll reaching a staggering 20 to 40 million worldwide.

Pandemics also occurred in 1957 and 1968 with the Asian flu and the Hong Kong flu, respectively.

Epidemics are widespread regional outbreaks that occur every two to three years and affect 5 to 10 percent of the population.

The Russian flu in the winter of 1977 is an example of an epidemic. A regional epidemic is shorter-lived than a pandemic and only lasts several weeks. Finally, there are smaller shoots each winter that are confined to specific locations.

The first extant descriptions of the flu were written nearly 2,500 years ago by the ancient Greek physician Hippocrates.

Historically, influenza has been attributed to several different agents, including “bad air” and several different bacteria.

In fact, its name comes from the Italian word for “influence”, because people in 18th century Europe thought that the disease was caused by the influence of bad weather.

It was not until 1933 that the causative agent was identified as a virus.

Virus types

There are three types of influenza viruses identified as A, B, and C.

Influenza type A usually infects a wide variety of animals, such as pigs, horses, and birds.

Humans are infected by all three types of viruses A, B, and C.

Influenza A is responsible for most cases of the flu, while infection with types B and C is less common and causes milder illness.

Seasonal epidemics are caused by influenza A and B viruses, since 1968, most of the reported seasonal influenza epidemics have been caused by influenza A: H3N2 virus.

Influenza B viruses can cause milder illness but often cause epidemics with moderate or severe illness, either as the predominant circulating virus or in conjunction with influenza A.

Causes of influenza

Influenza causes a sporadic illness that usually occurs during the fall and winter in temperate climates and is called seasonal epidemics.

Most influenza epidemics are caused by one predominant serotype, but different influenza viruses may appear sequentially in one location or they may appear simultaneously, with one virus predominant in one location and another predominant in another location.

Flu pandemics are less common.

As of 2018, there have been 6 major pandemics, usually named after the presumed location of origin:

  • In 1889: Russian influenza (H2N2).
  • In 1900: old Hong Kong flu (H3N8).
  • In 1918: Spanish influenza (H1N1).
  • In 1957: Asian influenza (H2N2).
  • In 1968: Hong Kong flu (H3N2).
  • In 2009: swine flu (influenza A [H1N1]).

Influenza viruses can be spread by:

  • Droplets in the air.
  • Person-to-person contact.
  • Contact with contaminated items.
  • Airborne spread appears to be the most important mechanism.

Morbidity and mortality in these patients may be due to exacerbation of an underlying disease, acute respiratory distress syndrome, primary influenza pneumonia, or secondary bacterial pneumonia.


Symptoms begin approximately 48 hours after exposure to the virus, although this period can vary from one to four days.

Usually the fever is continuous for about three days, although symptoms such as cough, lethargy, and a general feeling of being unwell may persist longer.

Most people have an uneventful recovery after treatment for symptoms alone.

A person is considered infectious when the virus is shed through the respiratory tract (nose and throat).

This can occur from the day of infection and can continue for eight days, or longer in people with lowered immunity.

When coughing or sneezing, the virus is transmitted in the form of drops, most of the time, these tiny infectious particles can remain suspended in the air for up to an hour, the virus can also remain on hands, tissues or other contaminated objects.

If other people breathe in the droplets or touch infected surfaces, the virus can reach cells in the airways and establish infection there.

Typical flu symptoms include an abrupt onset of a headache, dry cough, and chills, quickly followed by a general malaise and a fever that can reach 40 ° C (104 ° F).

Flu victims feel extremely tired and weak, and may not return to normal energy levels for several days or even a couple of weeks.

Headache in influenza is very severe, often with photophobia and retrobulbar pain.

As the fever subsides, nasal congestion and irritating sore throat, substernal burning, non-productive cough, and sometimes coryza become apparent.

Respiratory symptoms may be mild at first, later lower respiratory tract disease becomes dominant, cough may be persistent, raspy, and productive.

Gastrointestinal symptoms can appear and appear to be more common with the H1N1 pandemic strain.

Children may have prominent nausea, vomiting, or abdominal pain, and infants may have a sepsis-like syndrome.

The typical flu in adults is also characterized by sudden onset of chills, fever that can reach 40 ° C (104 ° F), prostration, cough and general pain, which is why it is usually confused with influenza.

Complications of influenza

Both children and adults can develop serious complications when they have the flu. The pneumonia severe pulmonary primary virus of influenza occurs when the flu has caused damage.

After the common flu symptoms, the fever persists, the cough worsens. In severe cases, you may develop a bluish tint to your skin and feel confused from a lack of oxygen.

This form of pneumonia is very serious and requires hospitalization.

Most people who develop it have underlying heart or lung disease.

Secondary bacterial pneumonia is more common, occurs when bacteria cause a secondary lung infection.

Symptoms include a recurrence of fever, shortness of breath, and a productive cough that lasts four to fourteen days after flu symptoms are almost gone.

Pneumonia is usually caused by the bacteria Streptococcus pneumonia, Staphylococcus aureus, and Haemophilus influenzae type B.

Some doctors recommend that people in high-risk groups get vaccinated against pneumococcal pneumonia and the flu.

Pneumonia is suggested by a worsening cough, bloody sputum, dyspnea, and rales.

Persistence or recurrence of fever and cough suggests secondary bacterial pneumonia after the primary disease appears to be resolving.

Viruses can worsen chronic lung diseases, such as chronic obstructive pulmonary disease.

Young children are prone to middle ear infections (otitis media) when they have an upper respiratory infection.

In most cases, the ear infection is caused by the virus itself and is not a secondary bacterial infection, so antibiotics are rarely necessary.

Children can also develop a viral infection of the vocal cords and major major airways as a result of flu and other respiratory tract virus infections.

In rare cases, mainly in children, inflammation of the muscles of the body (myositis) can occur.

Pain in the leg muscles is a symptom.

Also very rarely, the heart muscle can become inflamed (myocarditis).

Symptoms include tiredness, shortness of breath, heart palpitations, rapid pulse, and chest discomfort.

Because myositis and myocarditis are more likely if your muscles are under stress, it is not a good idea to exercise while you have the flu (or any viral illness).

Young children with the flu may have bouts of fever (febrile seizures).

Febrile children should never be given aspirin or medications that contain aspirin.

A disease that affects the brain and liver, Reye’s syndrome, can occur in children who receive aspirin for fever associated with viral infection.

Reye’s syndrome is characterized by encephalopathy, fatty liver, elevated liver enzymes, ammonia or both, hypoglycemia and lipidemia. It often occurs during epidemics of influenza B.

Symptoms include vomiting, lethargy, altered consciousness, seizures, and respiratory arrest.

Most recover, but it can sometimes lead to permanent brain damage or death.

Other very rare complications that occur in the central nervous system and are associated with influenza include Guillain-Barré syndrome, encephalitis, and transverse myelitis.


Doctors can usually recognize the flu by looking at the symptoms and signs on their own.

Other respiratory viruses can cause a similar clinical picture, but if influenza is known to be active in the community, then the diagnosis of influenza becomes even more likely.

Although there are specific viral culture tests available to identify the influenza virus strain from respiratory samples, results can take several days.

Therefore, physicians generally rely on a set of symptoms and the presence of influenza in the community for diagnosis.

Specific tests are helpful in determining the type of flu in the community, but they do little for individual treatment.

Doctors can administer tests, such as cultures of samples taken from the throat, to identify secondary infections.

A throat or nasal swab is taken and the virus is cell-cultured.

The virus can be detected in several ways, for example: adding red blood cells or using a fluorescence microscope.

Newly developed techniques can detect the presence of the genetic information (RNA) of the virus.

Pulse oximetry and chest radiography are usually recommended for patients with severe respiratory symptoms.

If patients have lower respiratory tract symptoms and signs, such as dyspnea and rales noted during lung examination, pulse oximetry should be performed to detect hypoxemia and a chest X-ray to detect pneumonia.

These tests are important because antiviral treatment is generally indicated, in addition, they can avoid the unnecessary use of antibacterial drugs, and the identification of the specific influenza virus can be important for the control of the infection in the community.

Influenza treatment

Essentially, an influenza attack must be allowed to run its course. Symptoms can be relieved with bed rest and staying well hydrated. A vaporizer can make breathing easier.

In low-risk cases, with no signs of secondary bacterial infection, the flu will be treated primarily with over-the-counter medications.

In pregnant women, caution should be exercised when taking medications, and children under the age of 16 should not take aspirin.

Such medications can help relieve symptoms, but they will not fight the virus or cure the flu:

  • Acetaminophen or ibuprofen can help relieve fever, muscle aches, and headaches.
  • Nasal decongestants can help, but using them for more than five consecutive days will make symptoms worse later, due to a rebound effect.
  • Cough suppressant mixes can help eliminate a dry cough.
  • Vitamin supplements, such as vitamin C and A, and zinc lozenges have been recommended to help treat colds and flu, but there is little evidence of their effectiveness. High doses of vitamin C and zinc can cause gastrointestinal side effects, such as stomach pain, nausea, and vomiting.
  • In vitro tests have shown that spirulina can prevent the influenza virus from reproducing.
  • Probiotics can help general immune function, but this effect has not yet been proven in the influenza virus.

In high-risk cases, a doctor may prescribe antiviral drugs:

  • The neuraminidase inhibitors oseltamivir and zanamivir are currently available for the treatment and prevention of influenza A and B. Due to the risk of severe bronchospasm, zanamivir should not be used by people with underlying respiratory disease.
  • Adamantanes, for the treatment and prevention of influenza A, are not recommended as the viruses have become resistant to them.

Preventing influenza with antivirals should not be viewed as a substitute for vaccination, and should be reserved for people who cannot be vaccinated or who are severely immunosuppressed.

Influenza should not be treated with antibiotics, as it is caused by a virus, not a bacteria.

Antibiotics should only be prescribed when a secondary bacterial infection is suspected.

In very severe cases, the doctor might consider hospitalization of the patient.

Special considerations

  • Influenza and breastfeeding: The flu virus cannot be transmitted from mother to baby through breast milk. Keep breastfeeding if you have the flu, as the antibodies you pass to your baby through your breast milk help protect your baby from infection.
  • Influenza and Pregnancy: Pregnant women, especially those in the second and third trimesters, are at increased risk of developing severe seasonal influenza. Current flu vaccination recommendations include all pregnant women.
  • Influenza and sports: refrain from strenuous exercise while sick. If you are a professional athlete, remember that various ingredients in over-the-counter cold and flu medications are prohibited by the respective governing bodies.
  • Flu and smokers: In smokers, the cilia in the airways that clean the lungs are already damaged, which means that an important defense mechanism of the airways is compromised. The flu can make one even more vulnerable to complications like a secondary bacterial infection.

Flu Prevention

Although the flu is generally not dangerous, it can cause serious complications and even death, especially in the elderly and very young.

You should try to avoid getting this highly contagious disease, especially if you are vulnerable.

You really can’t avoid being exposed to the cold and flu virus.

Many are acquired from people who do not yet show symptoms, and it is difficult to contain airborne viruses.

However, there are general steps you can take to give yourself the best chance of avoiding infection:

  • Wash your hands frequently and do not touch your nose, eyes, or mouth unnecessarily.
  • Contain sneezes and coughs with tissues, discarding them immediately and washing hands afterward.
  • Do not touch surrounding objects when in public places, such as an escalator rail, a pencil, or a coughing mouse.

To minimize exposure, close or prolonged contact with people with colds or flu should be avoided. With an incubation period of one to four days and a contagious period of seven days or more, it is best to avoid anyone with the flu for at least a week.

Vitamin A supplements can be used to prevent flu in malnourished children.

Quitting Smoking Smokers are more vulnerable to complications from respiratory infections.