Flu: everything you need to know about causes, symptoms and treatments

13 March 2024
Alexis Bikfalvi


Influenza is an infectious disease that affects millions of people worldwide every year. It is caused by viruses that are transmitted through the respiratory tract and lead to symptoms such as fever, cough, headaches, body aches, and fatigue. Influenza can range from mild to severe, depending on the type of virus, the individual’s health condition, and age. In some cases, influenza can result in serious complications, even death, such as pneumonia, sepsis, or acute respiratory distress syndrome.

In this article, we will explain what influenza is, how to recognize it, how to treat it, and how to prevent it. We will also discuss seasonal influenza and influenza vaccination. You will find useful information and practical tips to protect yourself and your loved ones from this disease.

What is the flu?


The flu is a viral infection that affects the upper and lower respiratory tract. It manifests as inflammation of the nasal mucosa, throat, and bronchi. The flu can be caused by different types of viruses, which belong to the family of Orthomyxoviridae. The most common ones are type A, B, and C viruses. Type A virus is the most dangerous as it can mutate rapidly and cause epidemics or pandemics. Type B virus is less common and less severe. Type C virus is the mildest and generally does not cause complications.

The causes of the flu

The flu is caused by inhaling or coming into contact with particles infected with the virus. These particles can be emitted by a sick person coughing, sneezing, or talking. They can also be found on contaminated surfaces or objects, such as door handles, keyboards, or phones. The virus then enters the body through the mucous membranes of the nose, mouth, or eyes. It multiplies in the epithelial cells of the respiratory tract and causes an inflammatory reaction. The immune system responds by producing antibodies and killer cells, which attempt to neutralize the virus. However, the virus can evade immune defenses by changing its structure. This is known as the virus’s antigenicity. There are two types of antigenic variations: mutation and reassortment.

Mutation is a random change in the genetic material of the virus. It can affect the surface proteins of the virus, called hemagglutinin (H) and neuraminidase (N). These proteins are responsible for attaching the virus to host cells and for its release. They are also recognized by antibodies. Mutation can make the virus more or less virulent and more or less resistant to antibodies. This is called antigenic drift. It frequently occurs in type A and B viruses, leading to the emergence of new virus strains. Reassortment is an exchange of genetic material segments between two type A viruses. It can happen when two type A viruses infect the same cell. It can give rise to a new virus with different characteristics from the parent viruses. This is called antigenic shift. It occurs rarely but can lead to pandemics, such as the Spanish flu of 1918, the Asian flu of 1957, the Hong Kong flu of 1968, or the H1N1 flu of 2009.

Disease Transmission

Influenza is primarily transmitted through respiratory droplets by inhaling infected droplets. These droplets can travel about 2 meters and remain suspended in the air for a few minutes. They can also land on surfaces or objects where the virus can survive for several hours. Influenza can also be transmitted through direct or indirect contact with a sick person or their secretions. For example, by shaking hands, kissing, sharing utensils, or touching used tissues.

The incubation period for influenza is 1 to 4 days, which is the time between infection and the onset of symptoms. The contagious period is 5 to 7 days, during which the person can spread the virus to others. Contagiousness starts one day before symptoms appear and ends when symptoms resolve. In children, the elderly, and immunocompromised individuals, the contagious period may be longer.

Influenza Symptoms

Common Symptoms

Influenza symptoms are generally easy to recognize as they appear suddenly and are quite intense. The common symptoms of influenza include:

  • High fever, ranging between 39°C and 40°C, accompanied by chills;
  • Dry and irritating cough, which can be painful;
  • Sore throat;
  • Headaches;
  • Muscle and joint aches, which can be diffuse or localized;
  • Intense fatigue, often making it difficult to get out of bed;
  • Loss of appetite;
  • General feeling of discomfort.

Influenza symptoms typically last about a week, but they can persist longer in certain individuals, especially the elderly, children, and those with weakened immune systems. Coughing and fatigue may continue for several weeks after recovery.

Complex Symptoms

In some cases, influenza can lead to more complex symptoms that require urgent medical attention. These symptoms may be signs of serious complications such as pneumonia, sepsis, meningitis, or myocarditis. The complex symptoms of influenza include:

  • Difficulty breathing, which can manifest as shortness of breath, rapid or wheezing breathing, cyanosis (bluish discoloration of the skin and mucous membranes), or chest pain;
  • Confusion, which can present as disorientation, drowsiness, speech difficulties, hallucinations, or seizures;
  • Dehydration, which can be recognized by intense thirst, dry mouth, decreased urine volume and frequency, dry and cold skin, sunken eyes, or cracked lips;
  • Worsening of a chronic condition such as diabetes, heart failure, kidney failure, asthma, or COPD;
  • Onset or increase of nasal discharge, sinus pain, ear pain, or purulent sputum (yellow or green phlegm);
  • Skin rash, which can be red, purplish, or petechial (small red or black dots);
  • Abdominal pain, nausea, vomiting, or diarrhea, especially in children.

If you experience one or more of these symptoms, you should seek medical attention promptly as it could be a life-threatening emergency. The doctor may order additional tests such as a chest X-ray, blood analysis, or nasal swab to confirm the diagnosis of influenza and assess the severity of your condition. They may also refer you to a hospital department appropriate for your situation.

Diagnosis of Influenza

When to Consult a Doctor?

Most cases of influenza are mild and do not require medical consultation. You can treat yourself at home by following the advice from the previous section. However, there are situations where it is recommended to consult a doctor, either to confirm the diagnosis of influenza or to prevent and treat complications. You should consult a doctor if:

  • You are part of the high-risk group for complications, such as individuals over 65 years old, pregnant women, children under 5 years old, individuals with chronic conditions (diabetes, asthma, heart failure, etc.), immunocompromised individuals (HIV, cancer, transplant, etc.), or obese individuals;
  • You experience complex symptoms such as difficulty breathing, confusion, dehydration, worsening of a chronic condition, skin rash, or signs of bacterial infection (nasal discharge, sinus pain, ear pain, purulent sputum);
  • You have had close contact with a person with avian or swine flu, or with an infected animal;
  • You have traveled to an area where there is an epidemic of influenza, particularly avian or swine flu;
  • You have doubts about the diagnosis of influenza, or you want to receive antiviral treatment.

If you consult a doctor, you should inform them that you have symptoms of influenza and wear a surgical mask to avoid contaminating others. You should also follow preventive measures such as washing your hands, coughing into your elbow, using single-use tissues, and avoiding touching common objects.

The medical diagnosis of influenza

The medical diagnosis of influenza is based on clinical examination and, if necessary, laboratory tests. The doctor will ask you about your symptoms, their onset, intensity, and progression. They will also examine you, especially your temperature, heart rate, respiratory rate, blood pressure, oxygen saturation, hydration status, level of consciousness, general condition, and lung auscultation. The doctor will also look for signs of complications such as pneumonia, sepsis, meningitis, or myocarditis.

The clinical diagnosis of influenza is often sufficient, especially during an epidemic when symptoms are typical and the patient has no risk factors. In such cases, the doctor may prescribe symptomatic treatment and advise you to stay at home until you recover. The clinical diagnosis of influenza has a sensitivity of 79% and a specificity of 65%, meaning there can be false positives (people diagnosed with influenza who do not have it) and false negatives (people who have influenza but are not diagnosed). Laboratory diagnosis of influenza is sometimes necessary, especially in the following situations:

  • The patient is hospitalized or requires hospitalization;
  • The patient has complex symptoms or severe complications;
  • The patient is at risk of complications;
  • The patient has been in contact with a person or animal with avian or swine flu;
  • The patient has traveled to an area with a flu epidemic, especially avian or swine flu;
  • The patient wants antiviral treatment;
  • The doctor has doubts about the diagnosis of influenza or wants to rule out other possible causes of fever;
  • The doctor wants to participate in influenza epidemiological surveillance.

Laboratory diagnosis of influenza involves taking a sample of respiratory secretions, either through a nasal or pharyngeal swab or through nasal or bronchoalveolar lavage. The sample is then analyzed using various methods, each with its advantages and disadvantages:

  • Molecular tests, which detect the virus’s genetic material, such as polymerase chain reaction (PCR) or other nucleic acid amplification tests. These tests are the most sensitive and specific as they can identify the virus type, subtype, and antiviral resistance. They can be performed at the point of care with rapid results or in a laboratory with longer results;
  • Antigen tests, which detect the virus’s surface proteins like hemagglutinin or neuraminidase. These tests are less sensitive and specific than molecular tests as they may not recognize certain virus strains or confuse the virus with other infectious agents. They are performed at the point of care with results in minutes;
  • Serological tests, which detect antibodies produced by the body in response to the virus. These tests are less useful for diagnosing influenza as they require two samples taken 15 days apart and cannot distinguish recent from past infections. They are performed in a laboratory with results in days.

The choice of laboratory test depends on the clinical situation, test availability, cost, and result turnaround time. Generally, molecular tests are preferred over antigen tests because they are more reliable and informative. Serological tests are rarely used except for research or epidemiological surveillance purposes.

Treatment for Influenza

Medication Treatments

The medical treatment for influenza aims to relieve symptoms and prevent or treat complications. There are two types of medications: antipyretics and antivirals.

Antipyretics are medications that reduce fever and alleviate pain. Paracetamol is the most commonly used antipyretic as it is effective and well-tolerated. It can be taken by most individuals, including pregnant women, children, and the elderly. It is important to follow the dosage instructions on the package or as advised by the doctor and not exceed 4 grams per day for adults and 60 mg per kilogram for children. Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen are other possible antipyretics, but they have more side effects and contraindications. Aspirin is not recommended for children as it can cause Reye’s syndrome, a rare but serious illness affecting the liver and brain. NSAIDs are not recommended for individuals with gastric or duodenal ulcers, kidney insufficiency, asthma, or allergies to these medications. They can also mask influenza symptoms and increase the risk of infectious complications.

Antivirals are medications that directly target the influenza virus by preventing its replication or release. They can reduce the duration and severity of symptoms as well as the risk of complications. They are reserved for individuals at risk of complications, those with severe influenza, or those exposed to the virus. They must be taken within 48 hours of symptom onset to be effective. There are two antivirals available in Switzerland: oseltamivir (Tamiflu) and zanamivir (Relenza). Oseltamivir is available as capsules or powder for oral administration. Zanamivir is available as a powder for inhalation. These antivirals can have side effects such as nausea, vomiting, headaches, digestive issues, or allergic reactions. They can also lead to virus resistance, limiting their effectiveness.

Natural Remedies

In addition to medical treatment, there are natural remedies that can help alleviate flu symptoms. These remedies are not scientifically validated but are based on folk experience or traditional medicine. They should not replace medical treatment or delay consulting a doctor in case of complex symptoms or complications. Here are some examples of natural remedies for the flu:

  • Honey, which has antibacterial, antiviral, and anti-inflammatory properties. It can be consumed pure or mixed with warm milk, tea, lemon juice, or apple cider vinegar. It can also be applied to the throat to relieve soreness. Honey should be avoided in children under one year old due to the risk of botulism;
  • Ginger, which has antiviral, anti-inflammatory, and expectorant properties. It can be consumed as a herbal tea by infusing fresh ginger pieces in boiling water or as a syrup by boiling grated ginger in sweetened water. It can also be added to dishes or fruit juices. Ginger should be avoided in case of coagulation disorders, gallstones, or pregnancy;
  • Garlic, which has antiviral, antibacterial, and immune-stimulating properties. It can be consumed raw, crushed, or chopped, or cooked by adding it to dishes. It can also be taken in the form of capsules or tincture. Garlic should be avoided in case of coagulation disorders, gastric or duodenal ulcers, or garlic allergy;
  • Echinacea, which has antiviral, immune-stimulating, and anti-inflammatory properties. It can be consumed as a herbal tea by infusing dried flowers or roots in boiling water or in the form of drops, tablets, or capsules. It can also be applied to the skin as a cream or ointment. Echinacea should be avoided in case of autoimmune disease, tuberculosis, leukemia, AIDS, or allergy to plants in the aster family;
  • Thyme, which has antiviral, antibacterial, expectorant, and antitussive properties. It can be consumed as a herbal tea by infusing dried leaves in boiling water or as a syrup by boiling fresh thyme in sweetened water. It can also be inhaled by adding a few drops of thyme essential oil to hot water or applied to the chest as a balm or poultice. Thyme should be avoided in case of pregnancy, breastfeeding, hypertension, hyperthyroidism, or thyme allergy.

Duration of the Illness

The duration of the flu varies depending on the individual and the type of virus. Generally, flu symptoms last about a week, but they can persist longer in certain individuals, especially older people, children, and fragile individuals. Coughing and fatigue can linger for several weeks after recovery. The contagious period of the flu is typically 5 to 7 days. This is the time during which a person can transmit the virus to others. The contagious period starts one day before symptoms appear and ends when the symptoms disappear. In children, older adults, and immunocompromised individuals, the contagious period can be longer.

Prevention of the Flu


Vaccination is the most effective way to prevent the flu and its complications. The flu vaccine contains inactivated fragments of influenza virus that stimulate the immune system to produce antibodies. These antibodies help recognize and neutralize the virus if an infection occurs. The flu vaccine is safe and well-tolerated. It may cause minor side effects such as pain, redness, or swelling at the injection site, or a moderate fever. Severe allergic reactions are very rare. The flu vaccine cannot cause the flu because it does not contain live viruses.

The flu vaccine needs to be renewed every year because the flu virus changes frequently. The composition of the vaccine is adjusted each year based on the circulating virus strains. The vaccine protects against the most common and dangerous strains of the virus. It does not offer 100% protection but reduces the risk of getting the flu, developing a severe form, or dying from it. The flu vaccine is recommended for people at risk of complications, such as those aged 65 and older, pregnant women, individuals with chronic illnesses, immunocompromised individuals, obese individuals, or children under 5 years old. It is also recommended for individuals in regular contact with high-risk individuals, such as healthcare professionals, caregivers, teachers, or personnel in healthcare or social care facilities. The flu vaccine is free for high-risk individuals and those in contact with them.

The flu vaccine should be administered before the start of the epidemic, preferably between October and December. It takes about 15 days after vaccination to be protected. The flu vaccine can be administered by a doctor, nurse, pharmacist, midwife, or physiotherapist. You need to present your coverage voucher sent by the Health Insurance, or your eligibility certificate, or your health insurance card. You also need to present your vaccination record, vaccination certificate, or certificate of contraindication.

Barrier gestures

Barrier gestures are simple prevention measures that help limit the transmission of respiratory viruses, such as the flu. These barrier gestures include:

  • Regularly washing hands with water and soap, or with a hydroalcoholic solution;
  • Wearing a surgical or fabric mask that covers the nose and mouth when sick or in contact with sick individuals;
  • Coughing or sneezing into your elbow or a single-use tissue and immediately disposing of it in a closed bin;
  • Avoiding touching your eyes, nose, or mouth, which are entry points for the virus;
  • Avoiding close contact with sick individuals, such as hugs, handshakes, or sharing personal items;
  • Ventilating and refreshing the air in enclosed spaces by opening windows for at least 10 minutes daily;
  • Cleaning and disinfecting potentially contaminated surfaces and objects, such as doorknobs, keyboards, phones, or toys.

Barrier gestures should be followed by everyone, even vaccinated individuals, as the flu vaccine does not provide total protection. They should be followed consistently, especially during epidemics, and reinforced when experiencing flu symptoms or in contact with high-risk individuals. Barrier gestures are an essential complement to vaccination to protect oneself and others from the flu and Covid-19.

Diet and Lifestyle Hygiene

A balanced diet and good lifestyle hygiene can contribute to preventing the flu by strengthening the body’s immune defenses. It is advisable to:

  • Consume fresh fruits and vegetables, which are rich in vitamins, minerals, and antioxidants;
  • Prioritize foods containing vitamin C, such as citrus fruits, kiwis, bell peppers, or broccoli;
  • Include foods containing zinc, such as oysters, shellfish, meats, eggs, legumes, or seeds;
  • Drink enough water, at least 1.5 liters per day, to avoid dehydration and promote toxin elimination;
  • Avoid alcohol, tobacco, coffee, and foods that are too fatty, sugary, or salty, as they can weaken the immune system;
  • Engage in regular physical activity, at least 30 minutes per day, to stimulate blood circulation and tissue oxygenation;
  • Get enough sleep, at least 7 to 8 hours per night, to allow the body to rest and regenerate;
  • Manage stress, which can impair the immune system’s function, by practicing relaxation techniques such as breathing exercises, meditation, or yoga.

A balanced diet and good lifestyle hygiene do not replace vaccination or barrier gestures, but they can complement them to protect against the flu and its complications.


Influenza is an infectious disease that can affect everyone but can be more severe in individuals at risk of complications. It presents with respiratory symptoms such as fever, cough, sore throat, or fatigue. It is transmitted through the air or by contact with contaminated persons or objects. It can be caused by different types of viruses that change frequently and can lead to outbreaks or pandemics.

The treatment of influenza focuses on relieving symptoms and preventing complications. There are antipyretic medications available that reduce fever and pain, as well as antiviral medications that act on the virus. Natural remedies are also available that can help improve symptoms. Diagnosis of influenza is typically based on clinical examination and sometimes laboratory tests. The duration of the illness is approximately one week but can be longer in some individuals. Prevention of influenza involves vaccination, which is the most effective way to protect oneself and others. The flu vaccine should be renewed annually due to virus evolution and is recommended for at-risk individuals and those in contact with them. Prevention of influenza also includes practicing barrier measures to limit virus transmission. These include hand washing, wearing masks, coughing into elbows, avoiding close contacts, ventilating spaces, and cleaning surfaces and objects. Additionally, maintaining a balanced diet and good hygiene are crucial in strengthening immune defenses. Influenza is a common but potentially dangerous disease, and it’s important to recognize, treat, and prevent it. If you have any doubts or questions, don’t hesitate to consult a doctor or pharmacist. Take care of yourself and your loved ones.



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