What is dangerous about influenza in adults: hidden complications and risks

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 constantly mutate and deceive the immune system. In adults, it often starts like a regular fever: body aches, headache, dry cough, weakness. But by the 3rd to 5th day, something more may begin — irreversible processes in the lungs, heart, and nervous system. Many believe that influenza will "pass on its own," especially if there is a "strong immune system." This is a dangerous misconception. In reality, it is adults, especially those over 40, for whom influenza more often leads to severe complications — not due to a weak body, but because the immune system reacts excessively, causing a cytokine storm, or, conversely, "falls asleep" under the pressure of the virus. And then even a healthy person can end up in intensive care within a week.

Classification of the disease according to ICD

In the International Classification of Diseases 11th Revision (ICD-11), influenza falls under the block "Infectious and parasitic diseases" (BA00–BA99), specifically — to code **BA20** — "Influenza." Under this code, all forms of the disease caused by influenza viruses A, B, and C are combined. Separate subcodes are highlighted:
— **BA20.0** — influenza caused by influenza virus A (including subtypes H1N1, H3N2, and others);
— **BA20.1** — influenza caused by influenza virus B;
— **BA20.2** — influenza caused by influenza virus C (rarely causes epidemics, usually has a milder course);
— **BA20.8** — other specified forms of influenza;
— **BA20.9** — unspecified influenza.
It is important to understand: the diagnosis of "influenza" in medical documentation must be confirmed by laboratory tests — clinically it can easily be confused with ARVI, adeno-, rhino-, or parainfluenza. That is why the ICD provides separate codes for "acute respiratory viral infection of unspecified origin" (BA10.9), to avoid diagnostic errors.

History of the disease and interesting historical facts

Influenza has been known to humanity for a long time. The first reliable description of an epidemic that meets modern criteria for influenza dates back to 1580 — it affected Spain, Italy, Russia, and Asia. But a real breakthrough in understanding came only in 1933, when British scientists William Smith, Christopher Andrews, and Patrick Laidlaw isolated the influenza virus A from the nasal mucus of an infected patient. This marked the beginning of the era of virology.
The most destructive pandemic of the 20th century — the "Spanish flu" of 1918–1919 — claimed between 20 and 50 million lives. Its peculiarity was that the highest mortality was observed among young adults aged 20–40 — not among the elderly or children. Modern studies have shown that the cause was a hyperreaction of the immune system (cytokine storm), rather than the virus itself directly. Interestingly, the name "Spanish flu" arose not because the epidemic started in Spain — the country was neutral in World War I and did not impose censorship on reports of the disease, unlike other states. Therefore, news of the scale of the epidemic first emerged from Madrid.
Another curious fact: in 1976, there was an outbreak of swine flu (H1N1) at the Fort Dix military base in the USA. Due to the fear of a repeat of the "Spanish flu," authorities launched a mass vaccination campaign — over 45 million people were vaccinated. However, the outbreak quickly subsided, and the vaccine was associated with an increase in cases of Guillain–Barré syndrome (autoimmune nerve damage). This case became an important lesson: even with a high threat, it is necessary to carefully weigh the risks and benefits of prevention.

Epidemiology: statistics on the occurrence of the disease

According to WHO, every year, between 51 million and 151 million people worldwide get infected with the flu — that is 290–650 million people. In seasonal years, the world records between 290,000 and 650,000 deaths from the flu and its complications. The most affected are people over 65 years old, children under 5 years old, and individuals with chronic diseases. But it is important: **in adults aged 30–60, the flu can also be fatal**, especially if there are accompanying factors.
In Russia, according to Rospotrebnadzor, in the 2023/2024 season, about 11.2 million cases of ARVI were registered, of which the flu was laboratory-confirmed in ~181 thousand cases (about 2 million). At the same time, the mortality from flu and pneumonia in the Russian Federation during the same months amounted to 14,321 cases — this is almost 3 times more than in the previous season. Why? Partly due to the decrease in collective immunity after three years of the COVID-19 pandemic, and partly due to the increase in the number of people with obesity, diabetes, and other risk factors.
Statistics on complications:
— Pneumonia — develops in 1–5% of adults with the flu, but in individuals over 65 years old — up to 15%;
— Exacerbation of COPD — in 20–30% of patients with chronic bronchitis;
— Cardiovascular events (heart attack, stroke) — the risk increases 6 times during the first week after infection;
— Myocarditis — occurs in 1–2% of hospitalized adults with severe flu.
These figures are not an abstraction. Behind every percentage are real people who could have avoided severe outcomes with timely treatment and prevention.

Genetic predisposition to influenza

Genetics plays a role, but not as directly as, for example, in hereditary diseases. Studies show that certain gene variants influence susceptibility to the virus and the severity of the disease. For example:
— The **IFITM3** gene (interferon-induced transmembrane protein 3) encodes a protein that blocks the entry of the virus into the cell. Carriers of the rs12252-C mutation have an increased risk of severe influenza — this variant is more common in Asian populations.
— Polymorphisms in the **TLR3**, **TLR7** genes (toll-like receptors) affect the recognition of viral RNA and the production of interferon. Defects here can lead to a weak primary response.
— The **CCR5** gene (chemokine receptor) — its deletion Δ32 is associated with resistance to HIV, but it can also affect the inflammatory response during influenza. Some data indicate a more severe course in carriers of this mutation during influenza A/H1N1.
However, it is important to emphasize: genetic predisposition is not a sentence. It merely modifies risk. The main determinants remain age, immune system status, presence of chronic diseases, and vaccination level. Even with an "unfavorable" genotype, timely prevention and treatment reduce the likelihood of complications to a minimum.

Risk factors for the occurrence and severe course of influenza in adults

Risk factors can be divided into **immutable** and **modifiable**. The former are those that are given at birth or related to age. The latter depend on lifestyle and individual choices.
Immutable:
— Age ≥65 years — decreased T-cell function and antibody production;
— Pregnancy (especially the II–III trimester) — immunosuppression and mechanical compression of the lungs;
— Congenital immunodeficiencies (e.g., IgA deficiency, agammaglobulinemia);
— Chromosomal abnormalities (Down syndrome — increased risk of pneumonia with influenza).
Modifiable:
— Smoking — damages the ciliated epithelium of the respiratory tract, reduces mucociliary clearance;
— Obesity (BMI ≥30) — adipokines provoke chronic inflammation, ventilation of the lungs is impaired;
— Type 2 diabetes — hyperglycemia suppresses phagocytosis and chemotaxis of neutrophils;
— Chronic lung diseases (COPD, bronchial asthma), heart (CHF, IHD), kidneys (CKD);
— Use of immunosuppressants (for example, after transplantation or in autoimmune diseases);
— Alcoholism — disrupts the barrier function of the mucosa and immune response;
— Malnutrition (deficiency of vitamins A, D, zinc, selenium).
Special attention — to people with "hidden" risks: for example, among 30% adults with normal weight, there is insulin resistance (prediabetes) that is undiagnosed but already reduces immune reactivity. If you need to assess your own risk — start with an analysis of the medical history and simple laboratory markers: fasting glucose, HbA1c, C-reactive protein, total immunoglobulin A.

Influenza diagnosis: how to distinguish it from ARVI and not miss the danger

The clinical picture of influenza in adults often begins suddenly: within 1–2 hours, the temperature rises to 38.5–40°C, there is a severe headache, muscle aches, weakness, and a dry cough. Unlike ARVI, a runny nose and sore throat in influenza are weakly expressed or appear later. The key symptom is **acute intoxication**: the person cannot get out of bed, loses appetite, and may have confusion (especially in the elderly).
Laboratory diagnostics:
— **Rapid antigen tests** (RIA, ELISA) — result in 15–30 minutes, sensitivity 50–70%, specificity >90%. A positive result is reliable, a negative one requires confirmation.
— **PCR diagnostics** — the "gold standard." Detects viral RNA in a nasopharyngeal swab. Sensitivity >95%. Conducted in specialized laboratories (for example, at the Central Research Institute of Epidemiology of Rospotrebnadzor).
— **Serology** (determination of IgM and IgG) — used retrospectively, during epidemiological investigations. Not suitable for emergency diagnosis.
Radiological methods are used only when complications are suspected:
— Chest X-ray — for cough >5 days, shortness of breath, decreased blood oxygen saturation;
— CT of the lungs — if the X-ray is uninformative, but the clinical picture is worsening (suspected viral pneumonia or bacterial superinfection).
Differential diagnosis includes:
— ARVI (adeno-, rhino-, parainfluenza);
— Covid-19 (symptoms overlap, PCR is needed);
— Bacterial pneumonia (high fever >5 days, purulent sputum);
— Infectious mononucleosis (enlargement of lymph nodes, spleen);
— Leptospirosis or other zoonoses with an epidemic history.
If you or your close one have a temperature >38.5°C + weakness + cough — do not wait "until tomorrow". The first 48 hours is the window for antiviral therapy. Delaying diagnosis by 1–2 days can cost lives.

Treatment of influenza in adults: what works and what is a myth

Influenza treatment is based on three pillars: **antiviral therapy**, **symptomatic support**, and **prevention of complications**. The main rule: antiviral medications should be started within the first 48 hours of symptom onset. After that, their effectiveness drops sharply.
Antiviral agents:
— **Oseltamivir** (Tamiflu) — a neuraminidase inhibitor. Taken orally, course of 5 days. Dose: 75 mg twice a day. Effective against viruses A and B.
— **Zanamivir** (Relenza) — the same mechanism, but inhalational. Not suitable for bronchospasm.
— **Baloxavir marboxil** (Xofluza) — inhibits the cap-dependent endonuclease of the virus. Single dose of 40 mg (for weight <80 kg) or 80 mg (≥80 kg). Especially effective in the first 24 hours.
Important: these medications **do not replace vaccination**, but help in case of an already started illness. They reduce the duration of fever by 1–2 days and lower the risk of hospitalization by 60%.
Symptomatic therapy:
— Antipyretics: paracetamol (max. 4 g/day) or ibuprofen (if there is no ulcer, renal failure). Aspirin is **contraindicated** for adults with influenza — risk of Reye's syndrome.
— Hydration: at least 2–2.5 liters of fluid per day (water, unsweetened compotes, electrolyte solutions in case of vomiting/diarrhea).
— Expectorants: ambroxol, acetylcysteine — for dry cough transitioning to productive.
— Oxygen therapy — for SpO₂ <94% (a pulse oximeter can be used at home for monitoring).
Surgical treatment is not directly applied for influenza, but may be required for complications:
— Drainage of the pleural cavity in case of empyema;
— Tracheostomy in case of severe respiratory failure;
— Removal of lung abscess in case of bacterial superinfection.
It is also important not to forget about non-drug measures: bed rest for the first 3-4 days, airing the room every 2 hours, using a humidifier (optimal humidity 40-60%). These simple steps speed up recovery and reduce the risk of secondary infection.

List of medications used for influenza in adults

Below is a table of the main medications with dosages, duration of use, and warnings. All medications should be prescribed by a doctor, especially in the case of chronic diseases.

Note: antibiotics for influenza **are not needed**, if there are no signs of bacterial superinfection (purulent sputum, fever >5 days, deterioration after temporary improvement). Self-administration of antibiotics leads to dysbiosis and resistance.

Disease monitoring: control stages, prognosis, complications

After the diagnosis of influenza, it is important not just to "take the pills," but to systematically monitor the condition. Here are the key monitoring points:
**Day 1**: temperature, respiratory rate, pulse, well-being. If the temperature does not decrease within 24 hours after starting oseltamivir — suspect bacterial superinfection or incorrect diagnosis.
**Day 3**: assess the cough — has it become productive? Is there shortness of breath while walking? Measure SpO₂ with a pulse oximeter. The norm is ≥95%TP3T. If below 94%TP3T — urgently see a doctor.
**Day 5**: if symptoms have not improved or have worsened — a consultation and chest X-ray are mandatory.
**Days 7–10**: monitor for residual effects — weakness, cough, sweating. In 10–15%TP3T of adults, post-influenza asthma or bronchitis develops, especially in smokers.
The prognosis depends on three factors:
— The timing of antiviral therapy initiation (the earlier, the better);
— The presence of comorbidities;
— Age and functional status of the immune system.
Complications are divided into **respiratory**, **cardiovascular**, **neurological**, and **metabolic**:
— Respiratory: viral pneumonia, bacterial pneumonia (often Streptococcus pneumoniae, Staphylococcus aureus), acute respiratory distress syndrome (ARDS);
— Cardiovascular: myocarditis, pericarditis, exacerbation of chronic heart failure, pulmonary artery thromboembolism;
— Neurological: meningitis, encephalitis, Guillain-Barré syndrome (rare, but possible after vaccination or infection);
— Metabolic: decompensation of diabetes mellitus, ketoacidosis.
Special concern is caused by **secondary bacterial pneumonia** — it develops in 10–20% of hospitalized adults and is the main cause of death from influenza. It is characterized by: a sharp deterioration after 3–5 days of "improvement," purulent sputum, leukocytosis >15×10⁹/l, focal opacity on X-ray.

Age-related features of influenza in adults

Influenza in adults is not a homogeneous phenomenon. Its course varies significantly depending on age and underlying health.
**Age 18–30 years**: usually mild or moderate course. High fever, pronounced intoxication, but rapid recovery. The risk of complications is low if there are no hidden pathologies (e.g., autoimmune diseases or HIV). However, myocarditis is more common in this group — due to an active immune response. Young people often ignore symptoms, continue to work and train — which exacerbates the load on the heart.
**Age 30–50 years**: here the share of "hidden" risks increases — obesity, arterial hypertension, prediabetes. Influenza can provoke acute heart failure or myocardial infarction. According to studies, the risk of hospitalization in 45-year-olds with a BMI ≥35 is 3 times higher than in their lean peers.
**Age 50–65 years**: age-related immunosenescence begins — antibody production decreases, T-lymphocyte function is impaired. Influenza lasts longer (7–10 days of fever), and bacterial infection is more often associated. Importantly: this group often does not have a pronounced fever — "atypical influenza" with predominant weakness and cough. This leads to late diagnosis.
**Over 65 years old**: the highest risk of death. In elderly patients, flu often begins without fever, but with confusion, weakness, and loss of appetite. Often, the first symptom is an exacerbation of COPD or heart failure. Mortality from flu in this group is up to 10% upon hospitalization. Key takeaway: **age alone does not make flu dangerous — accompanying conditions and delays in treatment are dangerous**. Even a 25-year-old athlete with chronic tonsillitis and smoking can end up in intensive care — if help is not sought in time.

Questions and answers: the most common inquiries about flu in adults

Question 1: Can flu be treated without medication, only with folk remedies?
No, it cannot — if we are talking about confirmed flu. Folk remedies (onion, garlic, honey, herbal infusions) can alleviate symptoms and support immunity, but they **do not suppress the virus**. Antiviral therapy is critical in the first 48 hours. Without it, the risk of complications sharply increases, especially in individuals over 40. If you decide to "wait it out," take a flu test — and if it is positive, consult a doctor. In 2024, free rapid tests will be available in clinics in Russia during the epidemic threshold.
Question 2: Why does cough and weakness persist for a long time after flu?
Cough persists due to damage to the respiratory epithelium. Cilia recover in 2–6 weeks — this is normal. But if the cough worsens, blood appears in the sputum, or shortness of breath occurs — consultation is needed. Weakness is related to the depletion of energy reserves and the inflammatory response. Cytokine levels (IL-6, TNF-α) remain elevated for another 2–3 weeks. Recommendations: gentle regimen, balanced nutrition (protein 1.2–1.5 g/kg body weight), vitamins B and D, physical therapy after normalization of temperature.
Question 3: Can flu cause a heart attack or stroke in a healthy person?
Yes, it can — even in those who had no complaints before. The virus causes systemic inflammation, which increases blood clotting and damages the endothelium of blood vessels. According to Lancet (2022), the risk of heart attack increases 6.2 times in the first 7 days after flu, and stroke — 3.1 times. The first 3 days are especially dangerous. Therefore, in adults over 40 with flu — monitor blood pressure, pulse, and at the slightest signs of rhythm or speech disturbances — call an ambulance immediately.
Question 4: Is it necessary to get vaccinated every year if last year's vaccine "worked"?
Yes, it is mandatory. Influenza viruses mutate — new strains circulate every season. The vaccine is updated annually according to WHO recommendations. Even if you did not get sick last year, it does not mean that immunity has been preserved: protection from vaccination lasts 6–8 months. Moreover, vaccination reduces the severity of the disease — even if you get infected, the risk of hospitalization decreases by 40–60%.

Typical mistakes in treating influenza and how to avoid them

1. **“I will recover at home — I won’t go to the hospital unnecessarily”**
→ Mistake: influenza progresses quickly. In adults over 30, deterioration can occur within 12–24 hours.
→ What to do: use a pulse oximeter (costs from 500 rubles), measure SpO₂ twice a day. If the number is ≤94% — call an ambulance. Don’t wait for “it to get really bad.”
2. **“I take antibiotics ‘just in case’”**
→ Mistake: antibiotics do not work on viruses. Taking them without indications leads to dysbiosis, allergies, and resistance.
→ What to do: a doctor prescribes antibiotics only when there is suspicion of bacterial superinfection (purulent sputum, fever >5 days, leukocytosis). Until then — only antivirals and symptomatic treatment.
3. **“I lowered my temperature — that means I’m recovering”**
→ Mistake: fever reduction masks symptoms, but the virus continues to multiply. A person gets up, goes to work — and provokes complications.
→ What to do: rest for at least 5 days from the onset of fever. Even if the temperature normalizes, the body is still restoring immunity. Return to activities gradually — starting from 30% of normal activity.
4. **“Vaccination is for the elderly and children”**
→ Error: adults aged 30–50 are one of the most vulnerable groups due to a “false sense of security.”
→ What to do: get vaccinated annually in September–October. Vaccines “Grippol Plus,” “Ultravaс,” “Vaxigrip Tetra” are available in Russia. All of them are quadrivalent and cover the current strains.

Conclusion: influenza is not a “cold,” but a serious threat to adults

Influenza in adults is not a reason to “stay under the blanket,” but a call to action. It is dangerous not so much in itself, but because of its hidden consequences: pneumonia, heart attack, myocarditis, exacerbation of chronic diseases. And the most frightening thing is that these complications often develop not immediately, but on the 4th–7th day, when a person already “feels better” and returns to work.
What really works:
— **Vaccination** is the best prevention (effectiveness 40–60% in the season, but reduces severity by 70%);
— **Early initiation of antiviral therapy** — within the first 48 hours;
— **Monitoring SpO₂ and temperature** — simple tools that save lives;
— **Avoiding self-medication with antibiotics and aspirin** — this is not “frugality,” but a risk.
If you are reading this article in the midst of the season — do three things now: check if you have oseltamivir in your medicine cabinet (or find out where to get it by prescription), buy a pulse oximeter, and sign up for vaccination. Don't wait for your neighbor to go to the hospital — your prevention starts today. Because the flu does not choose: it attacks everyone. But you can choose — to be prepared.