Flu (Influenza)

Flu (Influenza)


Influenza is a disease of the respiratory tract caused by infection with influenza viruses. These viruses cause damage to the mucous membrane of the respiratory tract, allowing viral toxins (poisons) or various bacteria to enter the body.

Influenza is a highly contagious disease and often occurs during the flu season (winter in the northern hemisphere). Due to the high infectiousness, so-called influenza epidemics occur sporadically (above-average number of influenza cases in a defined area in a certain period of time).

According to the German Robert Koch Institute, about five to twenty percent of the population become infected with the influenza virus during the winter flu wave every year. However, the extent of the flu waves varies greatly from year to year.

Since the Spanish flu in 1918/19, which claimed more than 20 million lives, flu pandemics (spread of an infectious disease across countries and continents) have occurred approximately every ten to fifteen years.

In the autumn of 1997 and again in 2004, an outbreak of avian influenza occurred in Asia, resulting in the transmission of the H5N1 influenza virus from birds to humans. However, a mass slaughter of poultry has so far prevented a pandemic. Nevertheless, according to experts, there is a danger that the virus could change in such a way that it could be transmitted directly from person to person. This change would greatly increase the risk of a pandemic.

The first outbreak of swine flu occurred in April 2009. This was triggered by the influenza virus A/H1N1. The great danger is that the H1N1 virus has already changed to such an extent that it no longer passes from pigs to humans, but is transmitted directly from person to person.


Influenza is caused by influenza viruses, which are mostly transmitted as droplet infections. Virus particles are thrown into the air when talking, sneezing or coughing and then inhaled by other people.

The incubation period (time from infection to the onset of symptoms) is between one and five days. Even during this symptomless period, infected people can infect other people. About seven days after the onset of the disease, the risk of infection is usually over.

Influenza viruses can be divided into the following types:

  • Influenza A virus: This type of influenza virus most commonly causes epidemics and pandemics. Because the virus can undergo rapid genetic change (known as antigenic drift), the body's immune system does not provide long-term protection against recurrence, so epidemics occur about every three years. If the influenza A virus mixes with animal influenza viruses, an even more pronounced genetic change occurs (called antigenic shift). No individual in the population now has immunity to this new, altered viral variant, which can lead to the development of an influenza pandemic. Examples of an antigenic shift include H5N1 avian flu and H1N1 swine flu.
  • InfluenzaB virus: Influenza caused by influenza B viruses occurs primarily in children and adolescents and is usually quite mild.
  • Influenza C virus: This virus type does not play a relevant role in influenza illnesses.

Children, the elderly and people with immunodeficiency are particularly at risk of developing severe, life-threatening complications in the course of an influenza illness.


Influenza illness is usually acute (sudden onset) and is accompanied by high fever, aching limbs and headache, and a feeling of fatigue. The recovery process can last up to several weeks in some cases, during which patients still suffer from an agonizing irritating cough. The disease severely weakens the immune system, making the body more susceptible to other serious infections.

Characteristic features of influenza are

  • sudden onset of illness
  • very high fever (often over 39°C) with chills
  • aching limbs and muscles
  • Feeling of weakness, fatigue and marked tiredness
  • headache
  • Sore throat and difficulty swallowing
  • dry, irritating cough with thick mucus
  • sensitivity to light, burning eyes
  • in some cases nausea

Like influenza, the common cold is caused by viruses, but the virus types are different. In contrast to the flu, the symptoms of influenza usually start suddenly and are much more pronounced.


Normally, influenza is diagnosed by the doctor on the basis of characteristic symptoms such as a sudden onset of high fever, severe fatigue and headache, aching limbs and muscles. It is important to distinguish the higher-risk flu illness from flu-like infections.

Up to three days after the onset of influenza, the influenza virus can be detected in secretions from the nasopharynx using a rapid test. In addition, it is also possible to cultivate the virus in cell cultures - however, this procedure is very time-consuming and only useful shortly after the onset of the flu. A blood test can also be helpful in making a diagnosis.


In most cases, flu is treated only symptomatically to relieve symptoms such as fever and pain. In people with a good immune system, this can successfully fight the viruses even without medical support.

However, if risk groups (children, elderly people or immunocompromised persons) are affected, certain drugs against the influenza virus are administered immediately. These are so-called neuraminidase inhibitors - these block the viral enzyme neuraminidase, which plays a major role in virus replication.

With the help of neuraminidase inhibitors, the course of the disease is shorter and milder. However, it is important that the drugs can only take effect properly if they are used within 48 hours of the onset of the disease. Accompanying symptoms such as pain or fever can be alleviated with other pain-relieving medications.

Because influenza severely weakens the body's immune system, there is a risk of secondary infection by bacteria. This can lead to serious illnesses such as pneumonia. Affected patients are then given an additional antibiotic to fight the bacterial infection.

To support and accelerate the healing process, the following measures help:

  • Adherence to bed rest and physical rest until the disease is completely healed.
  • Drinking plenty of fluids, as the body has a high fluid requirement due to the fever.
  • Take antipyretic medication or use antipyretic methods (e.g. calf compresses).
  • Consult your family doctor, especially if the symptoms do not improve after two days.
  • If, during the illness, you experience severe sore throat or earache, a sore irritating cough, difficulty breathing, purulent sputum or a fever over 39°C, contact a doctor immediately.
  • Refrain from smoking.


The flu usually runs its course without complications in young, otherwise healthy people.

Children, older people and people with immune deficiencies, however, have a higher risk of developing complications in the course of the disease. Secondary infection with bacteria or other viral pathogens can occur, causing the following serious illnesses:

  • Pneumonia (inflammation of the lungs)
  • Sinusitis (inflammation of the sinuses)
  • Otitis (ear infection)
  • Myocarditis (inflammation of the heart muscle)
  • Meningoencephalitis (inflammation of the meninges and brain)
  • Inflammation of the nervous system

Since these secondary diseases represent a life-threatening danger for risk groups (children, elderly people), a preventive influenza vaccination is recommended for these people. If influenza nevertheless occurs, drug therapy should be initiated as soon as possible to prevent complications.


Vaccination against influenza viruses is a good way to prevent contracting the flu. Ideally, this should be carried out annually between September and November. In the event of a flu epidemic, a later vaccination can still be useful.

Vaccination is recommended for the following risk groups:

  • everyone after the age of 60
  • people with immune deficiency (for example after organ transplantation or HIV infection)
  • pregnant women from the second trimester onwards
  • people suffering from chronic diseases such as lung diseases, cardiovascular diseases or diabetes mellitus
  • residents of old people's and nursing homes
  • People who have a lot of contact with other people and therefore have a higher risk of infection (e.g. teachers, police officers, doctors, nursing staff).

The vaccination protection does not take effect until about two weeks after the influenza vaccination. The vaccination is usually very well tolerated. In some cases, influenza is contracted despite vaccination, but this is much weaker and causes fewer symptoms. To provide reliable protection, the vaccination must be refreshed annually.

People who have an allergic reaction to chicken egg protein (chicken proteins) should not use the usual influenza vaccine - this can sometimes contain traces of chicken proteins due to its manufacturing process. Since 2007, other influenza vaccines have also been available which are produced using cell cultures and therefore do not contain chicken egg protein.

Editorial principles

All information used for the content comes from verified sources (recognised institutions, experts, studies by renowned universities). We attach great importance to the qualification of the authors and the scientific background of the information. Thus, we ensure that our research is based on scientific findings.
Danilo Glisic

Danilo Glisic

As a biology and mathematics student, he is passionate about writing magazine articles on current medical topics. Due to his affinity for facts, figures and data, his focus is on describing relevant clinical trial results.

The content of this page is an automated and high-quality translation from DeepL. You can find the original content in German here.



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