Flu and immunity: how the virus suppresses the body's defenses

Influenza is not just a "cold with a fever." It is an acute viral respiratory disease caused by RNA viruses of the genus *Influenzavirus*, which literally hack the body's defense mechanisms within hours. Within 24–48 hours after the virus enters the nasopharynx, you may feel muscle aches, a fever of 39–40 °C, headaches, weakness, a dry cough, and a sensation as if your body has been "taken apart." But the most dangerous aspect is not these symptoms, but how the virus suppresses the immune system: it does not just disguise itself from it, it actively blocks key links in the defense, making the body vulnerable to bacterial invasions and even its own cells. This is why influenza can lead to pneumonia, myocarditis, or exacerbation of chronic diseases—especially in those whose immune system is already "overworked."

Classification of the disease according to ICD-11

According to the International Classification of Diseases 11th Revision (ICD-11), influenza falls under the category "Infectious and parasitic diseases" (Block BA), subcategory "Viral infections of the respiratory tract" (Code BA42). The specific code depends on the type of virus and clinical form:

• BA42.0 — Influenza caused by influenza virus A (e.g., H1N1, H3N2)
• BA42.1 — Influenza caused by influenza virus B
• BA42.2 — Influenza caused by influenza virus C (rarely causes epidemics)
• BA42.3 — Influenza caused by influenza virus D (mainly in animals, rare cases in humans)
• BA42.Y — Other specified forms of influenza (e.g., mixed infection A+B)
• BA42.Z — Unspecified influenza

Important: ICD-11 introduces a division by severity—mild, moderate, severe, and critical forms. The critical form includes the development of acute respiratory failure, septic shock, or multiple organ failure. This is not just a formality—such codes directly affect treatment tactics and hospitalization. For example, with code BA42.0+BA42.Z (unspecified influenza with complications), the patient must be immediately referred to an infectious disease department, rather than treated on an outpatient basis.

History of the disease and interesting historical facts

Influenza has been known to humanity since ancient times. The first reliable description of an epidemic appears in the works of Hippocrates (5th century BC)—he described "fever with cough and shortness of breath" that struck Northern Greece. But a true breakthrough in understanding came only in the 20th century.
In 1918, the "Spanish flu" broke out—a pandemic of influenza A(H1N1) that claimed the lives of 50 to 100 million people worldwide. Notably, this wave had the highest mortality rate among the 20–40 age group—usually the most resilient to infections. Research from 2005, based on samples extracted from bodies buried in the permafrost of Alaska, confirmed that the virus had a unique ability to cause a "cytokine storm"—a hyperreaction of the immune system that killed not the virus, but the patient themselves.
Another interesting fact: in 1933, British scientists William Smith, Christopher Andrews, and Patrick Laidlaw first isolated the influenza virus A from the nasal mucus of a patient. They used chicken embryos for this— a method that remains the basis for vaccine production to this day. And yes, the name "Spanish flu" did not arise because it started in Spain—Spain was neutral in World War I and did not censor news about the disease, unlike the warring countries. Therefore, the first reports of mass cases came from Madrid—and the world mistakenly attributed the epidemic to this country.

Epidemiology: statistics on the occurrence of the disease

According to WHO, influenza affects between 51 million to 151 million people worldwide each year. This amounts to 290–650 million cases annually. Of these, about 3–5 million are severe forms, and 290–650 thousand end in death. The highest risk is among children under 5 years old, people over 65 years old, and individuals with chronic diseases.
In Russia, the flu season traditionally begins in November–December and peaks in January–February. According to Rospotrebnadzor for the 2024/2025 years, the epidemic threshold was exceeded in 47 regions of the country, with 12 of them exceeding it by more than 2 times. The most active strain was A(H3N2), which is characterized by a high mutation rate and frequent complications in the elderly.
Here is how cases are distributed by age groups (data for 2023, FGBU Research Institute of Influenza of the Russian Academy of Sciences):

Note: the numbers may seem modest, but considering that there are 146 million people in Russia, even 5% means 7.3 million cases per year. And this is only the registered cases. Many people experience influenza "on their feet," without consulting a doctor — the real number may be 30–50% higher.

Genetic predisposition to influenza

Genetics plays a role not in whether "you will get sick," but in "how you will cope with the illness." Research over the last 10 years has identified several key genes that influence susceptibility to influenza and the severity of the disease:

• IFITM3 (interferon-induced transmembrane protein 3) — this gene encodes a protein that blocks the virus from entering the cell. People with the rs12252-C mutation have an increased risk of severe influenza. This mutation occurs in 25% of Asians and only in 4% of Europeans.
• HLA-DRB1*07:01 — variant of the major histocompatibility complex gene. Carriers of this allele are worse at recognizing viral peptides, so their immune response is delayed.
• MBL2** (mannose-binding lectin 2)** — regulates innate immunity. MBL deficiency increases the risk of secondary bacterial infections during influenza.
• TLR7 — gene of the receptor that recognizes viral RNA. Mutations here are more common in women and are associated with increased interferon production — which is theoretically good, but can provoke a cytokine storm during influenza.

Important: genetic predisposition is not a death sentence. It only changes the "initial conditions." For example, a person with an IFITM3 mutation can avoid severe influenza if vaccinated on time and does not come into contact with the source of infection at the peak of the epidemic. Genetics is not fate, but a risk map that can be adjusted through prevention.

Risk factors for flu occurrence

Risk factors are divided into unmodifiable and modifiable. The former are things you cannot change (age, gender, genetics). The latter are those over which you have control.
Immutable:

• Age over 65 or under 2 years
• Gender: women in menopause and men with low testosterone levels have an increased risk of severe illness.
• The presence of chromosomal abnormalities (for example, Down syndrome - the risk of hospitalization is 10 times higher).

Modifiable:

• Smoking - reduces the function of ciliated epithelium in the airways by 30–50%. Even passive smoking increases the risk of infection by 20%.
• Chronic vitamin D deficiency. - a level below 20 ng/ml is associated with a 2-fold increase in the risk of hospitalization for influenza (according to a meta-analysis from 2022, BMJ).
• A body mass index of less than 18.5 kg/m² or obesity class III (BMI ≥ 40). - both conditions disrupt immune homeostasis.
• Stress and sleep disturbances. - with 6 hours of sleep per day, interferon production decreases by 30% compared to 8 hours.
• Air pollution (PM2.5). - every 10 µg/m³ increases the risk of hospitalization by 1.5% (data from the WHO European Region).

If your goal is to reduce risk - start not with vitamins, but with analyzing these factors. For example: do you smoke and live in a metropolis? Then even vaccination will not provide 100% protection - a comprehensive approach is needed.

Diagnosis of influenza: how to distinguish it from ARVI and other diseases

The main thing is not to confuse influenza with a common cold. Symptoms overlap, but there are "red flags":

• Sudden onset. - temperature spikes to 39–40 °C within 2–4 hours (with ARVI - gradually, over 1–2 days).
• Systemic symptoms dominate. - aches, headache, weakness are stronger than runny nose and cough.
• Runny nose - not always present. - in 30% of patients, it is completely absent, especially in the first 2 days.
• Cough - dry, paroxysmal, without sputum for the first 3-4 days

Laboratory diagnostics:

• Rapid antigen tests (RIA, ELISA) - sensitivity 50-70%, specificity 90-95%. Result in 15 minutes. Suitable for mass screening, but not for confirmation in severe patients.
• PCR test from nasopharyngeal and oropharyngeal swabs - "gold standard". Sensitivity >95%, allows to determine the type and subtype of the virus (A/H1N1, B/Victoria, etc.). Time - 4-6 hours.
• Serology (ELISA for antibodies) - used retrospectively: a fourfold or greater increase in IgG titers in paired sera confirms infection. Not suitable for emergency diagnosis.

Radiological methods are used only when complications are suspected:

• Chest X-ray - in case of suspected pneumonia (areas of opacity, "ground-glass" appearance)
• CT of the lungs - in unclear clinical situations, suspicion of abscess or pleural effusion

Differential diagnosis includes:

• ARVI (adenovirus, rhinovirus) - slow onset, fewer systemic symptoms
• COVID-19 - loss of smell/taste, more pronounced shortness of breath, but similar X-ray picture
• Bacterial pneumonia - temperature persists for >5 days, purulent sputum
• Mononucleosis (Epstein-Barr) - lymphadenopathy, tonsillitis with coating, leukopenia

Treatment of influenza: when and how to act

Treatment of influenza - it is not "recovery in 3 days," but management of the process to prevent complications. Main principles:
1. Antiviral therapy - only in the first 48 hours.
After 48 hours, effectiveness drops sharply, but it is still prescribed in severe cases. Drugs of choice:

• Oseltamivir (Tamiflu) - neuraminidase inhibitor. Dose: 75 mg twice a day for 5 days. In children - by weight (3 mg/kg). Contraindicated in severe renal failure.
• Zanamivir (Relenza) - also a neuraminidase inhibitor, but in the form of inhalations. Not suitable for bronchial asthma and COPD in history.
• Baloxavir marboxil (Xofluza) - inhibits cap-dependent endonuclease of the virus. Single dose (40 mg for adults, 20 mg for children ≥12 years). Effective even at 48-72 hours, but more expensive.

Symptomatic therapy — with common sense.
There are many mistakes here. For example:

• Paracetamol — safe at a temperature >38.5 °C. Maximum 4 g/day for adults.
• Ibuprofen — can be used, but not in the presence of a stomach ulcer or kidney failure.
• Aspirin — prohibited for children under 15 years due to the risk of Reye's syndrome.
• Antitussives (codeine, butamirate) — only for dry, exhausting cough. Contraindicated for wet cough!
• Decongestants (xylometazoline) — no more than 5 days, otherwise — drug-induced rhinitis.

Support for immunity — not "vitamins in cubes".
There is no evidence that vitamin C at a dose of 1 g/day speeds up recovery from influenza. But there is data on zinc: taking 75 mg/day (as acetate or gluconate) in the first 24 hours reduces the duration of symptoms by 1–2 days. The main thing is not to exceed 100 mg/day, otherwise — nausea and decreased immunity.

List of medications used for influenza

Here is a summary table of medications with dosage regimens and restrictions:

Note: antibiotics for influenza **are not prescribed prophylactically**. They are needed only in case of confirmed bacterial superinfection (for example, with the growth of Streptococcus pneumoniae in sputum or in X-ray pneumonia).

Disease monitoring: control stages and prognosis

Influenza requires monitoring not only in the acute period but also in the recovery phase. Control points:

• Day 1–2 — assessment of the effectiveness of antiviral therapy: temperature should decrease by 0.5–1 °C/day. If not — reconsider the diagnosis or dosage.
• Day 3–4 — appearance of sputum (transition from dry to wet cough) — normal. If the cough worsens and the temperature returns — bacterial pneumonia may be possible.
• Day 5–7 — assessment of lung function: shortness of breath when walking 50 m, blood oxygen saturation (SpO₂) should be ≥95% at rest.
• Day 10+ — monitoring of general condition: fatigue, dizziness, sweating may indicate post-influenza asthenic syndrome or myocarditis.

Forecast:

• Mild form — recovery in 7–10 days
• Moderate — 10–14 days, residual effects possible (cough, weakness)
• Severe — up to 21 days, risk of complications 15–20%
• Critical — mortality up to 30% even with intensive therapy

Frequent complications:

• Secondary bacterial pneumonia — most often Streptococcus pneumoniae, Staphylococcus aureus (including MRSA)
• Myocarditis and pericarditis — especially in young people without chronic diseases
• Acute respiratory distress syndrome (ARDS) — during cytokine storm, requires mechanical ventilation
• Exacerbation of chronic diseases — COPD, bronchial asthma, heart failure

Age-related features of influenza progression

Influenza "plays" by different rules depending on age. Here's how it looks in practice:
Children under 2 years:
Often there is no classic fever — instead, there is lethargy, refusal to eat, vomiting, diarrhea. The risk of complications is high: from otitis (up to 30% of cases) to bronchiolitis. Special attention to signs of respiratory failure: nasal wings "flare," retraction of intercostal spaces, respiratory rate >50 per minute in infants.
Children 2–12 years:
Characterized by high fever (up to 40.5 °C), vomiting, seizures (with rapid rise). Often develops "influenza meningism" — neck syndrome without meningitis. Important: in children under 5 years, the risk of Reye's syndrome with aspirin intake is almost 100% with influenza.
Adolescents and young adults (13–40 years):
Most often — a typical picture: acute onset, body aches, cough. But in this group, cytokine storm is possible — especially with H1N1. Symptoms: sharp deterioration on the 4th–5th day, shortness of breath, cyanosis, confusion. Requires immediate hospitalization.
Adults aged 40–65:
The risk of complications increases every year. Comorbidities are especially dangerous: hypertension, diabetes, obesity. In men over 50, it often masquerades as a "cold," but if the fever lasts >5 days — it's a warning sign.
Elderly (65+):
The temperature may be subfebrile (37.5–38.5 °C), but the condition is severe: confusion, adynamia, decreased blood pressure. In 40% — the first symptom becomes cognitive impairment. Mortality from influenza in this group is 10 times higher than average.

Questions and Answers

Question 1: Can influenza be treated at home without doctors?
Yes, but only in mild cases in healthy individuals under 65 years. Conditions: temperature below 39 °C, no shortness of breath, cough not worsening, SpO₂ ≥95%. Mandatory: antiviral within the first 48 hours, temperature monitoring, hydration (2.5–3 l/day), rest. If there is no improvement by the 3rd day — seek help. Self-medication is dangerous with chronic diseases, pregnancy, age >65.
Question 2: Why does a cough persist for a long time after the flu?
This is not a "residual virus," but damage to the respiratory epithelium. Ciliated epithelium recovers in 2–4 weeks. During this time, reflex sensitivity is heightened — any irritant (cold air, smoke) causes coughing. Helpful: humidifying the air (50–60% humidity), ambroxol, avoiding irritants. If cough lasts >4 weeks — X-ray is needed to rule out bronchitis or tuberculosis.
Question 3: Does vaccination help if already in contact with an infected person?
Yes, but not as a "medicine." Vaccination after contact will not prevent the disease, but may soften its course and reduce the risk of complications. Especially important for at-risk individuals. However, if you already feel symptoms — vaccination should not be given, only antiviral.
Question 4: Can influenza cause a heart attack?
Yes. According to a study in Lancet (2020), the risk of acute myocardial infarction within 1 week after influenza increases by 6 times. Mechanism: inflammation enhances atherosclerotic plaques, increases blood clotting, causes tachycardia. In people with coronary artery disease, influenza is a direct trigger for a cardiac event.

Typical mistakes in treating influenza and how to avoid them

• Error: "I will survive — I have a strong immune system"
  → Reality: immunity is not "strong" or "weak" — it is specific. The influenza virus can suppress the interferon response. Even an athlete can get pneumonia.
  How to avoid: get vaccinated annually, do not ignore symptoms, especially in the first 2 days.
• Error: taking antibiotics "just in case"
  → Reality: antibiotics do not work against viruses. Their uncontrolled use leads to dysbiosis, resistance, and increases the risk of secondary infection.
  How to avoid: an antibiotic is prescribed only by a doctor upon confirmation of a bacterial infection (sputum analysis, X-ray).
• Error: "I will drink vitamin C and everything will pass"
  → Reality: vitamin C does not reduce the risk of influenza and does not speed up recovery in adults (Cochrane meta-analysis, 2013).
  How to avoid: focus on sleep, hydration, zinc, and antivirals — not on "magic pills."
• Error: continuing to work at a temperature of 38.5 °C
  → Reality: exertion during influenza increases the risk of myocarditis. Even in young people — fatalities are recorded after physical activity against the background of infection.
  How to avoid: at least 5 days of rest, even if you "feel better." The body restores immune memory — this requires energy.
• Error: using folk remedies instead of antivirals
  → Reality: tea with honey and lemon is a good support, but not a substitute for oseltamivir. In severe cases, a 24-hour delay in therapy increases mortality by 15%.
  How to avoid: use folk remedies as a supplement, not a foundation. And only after consulting a doctor.

Influenza is not "just a virus," but a complex battle between the pathogen and the immune system, where victory depends not on strength, but on the accuracy and speed of response. The influenza virus has evolved to suppress key defense links: it blocks interferon production, disguises its RNA as "self," and even forces immune cells to attack their own tissues. But we have weapons: vaccines, antivirals, knowledge of risk factors, and understanding when to stop and seek help. The main thing is not to underestimate the disease and not to overestimate your strength. Prevention today is not fear, but a conscious choice. And remember: the most reliable shield is not in the pharmacy, but in your sleep schedule, vitamin D levels, and determination to get vaccinated before the start of the season.