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. Within 1-2 days after infection, a person becomes a source of infection — even before the first symptoms appear. In Spain, influenza affects from 5% to 15% of the population annually, and in epidemic seasons — up to 20%. It is especially dangerous for the elderly, children under 5 years old, and those with chronic diseases: asthma, diabetes, heart failure. But the biggest problem is not the severity of the disease in one person, but its ability to spread quickly through offices, schools, public transport, and even in the elevators of residential buildings. That is why knowing how not to infect others during influenza is not a matter of politeness, but an element of public safety.
Classification of the disease according to ICD-11
In the International Classification of Diseases 11th Revision (ICD-11), adopted by WHO and in effect in Europe since 2022, influenza is coded as **1A00** — "Influenza caused by influenza virus A" and **1A01** — "Influenza caused by influenza virus B." There are also subcodes for clarification:
— **1A00.0** — influenza A, without specifying the strain;
— **1A00.1** — influenza A(H1N1)pdm09 (“swine flu”);
— **1A002** — influenza A(H3N2);
— **1A01.0** — influenza B, lineage Victoria;
— **1A01.1** — influenza B, lineage Yamagata (although this lineage, according to ECDC data, has not circulated since 2020).
Important: in ICD-11, influenza is never classified as “acute respiratory viral infections” (ARVI) — it is a separate nosological group. ARVI is a collective term for all other viruses (adenoviruses, rhinoviruses, parainfluenza, etc.), while influenza is strictly *Influenzavirus*. This distinction is critical for the prescription of antiviral drugs and epidemiological surveillance.
History of the disease and interesting historical facts around the world
Influenza has been known to humanity since ancient times, but the first reliable descriptions date back to the Middle Ages. In 1580, the first documented pandemic occurred in Europe — it started in Asia, passed through Africa, and reached Spain in 3 months. At that time, doctors referred to it as “chest fever” because the main symptom — severe bronchospasm and shortness of breath — was more noticeable than fever.
The most devastating pandemic — the “Spanish flu” of 1918–1919 — actually did not originate in Spain. The name stuck because Spain was a neutral country during World War I and did not censor news about the disease. In other countries, information was concealed — to avoid undermining morale. According to WHO estimates, 50 to 100 million people died over 2 years — more than in the entire war. Notably, this pandemic had a high mortality rate among young healthy people aged 20–40, rather than the elderly. Scientists still debate: possibly due to the “cytokine storm” — when the immune system overreacts and attacks its own lungs.
Another interesting fact: in 1976, influenza A(H1N1) resembling the 1918 strain broke out at Fort Dix military base in the USA. The government launched a mass vaccination campaign — but a few weeks later it turned out that the epidemic had subsided on its own. More than 400 people developed Guillain-Barré syndrome after vaccination. This case became a turning point in the history of vaccination: now all new vaccines undergo multi-stage monitoring before mass use.
Epidemiology worldwide: statistics on the occurrence of the disease
According to the European Centre for Disease Prevention and Control (ECDC), seasonal influenza in Europe leads to:
— 40,000–70,000 deaths per year;
— 2–5 million hospitalizations;
— loss of 20–30 million workdays.
In Spain, according to the Ministry of Health (Ministerio de Sanidad), the average seasonal rate:
— 150–250 cases per 100,000 population;
— peak activity — from December to March;
— in the 2022/23 year, 187,000 laboratory-confirmed cases were registered (an increase of 40% compared to the previous season).
Interestingly, the flu "does not like" the tropics: it circulates year-round there, but without a clear peak. In temperate latitudes — like in Spain — seasonality is related to low humidity in winter and increased time spent indoors. Studies show: at relative humidity below 40%, viral droplets remain in the air longer, and the nasal mucosa dries out — reducing the protective function of the ciliated epithelium.
It is worth noting: from 2020 to 2022, flu epidemics almost disappeared — not due to vaccines, but due to quarantine measures, masks, and social distancing. In 2023, when restrictions were lifted, Europe saw a "resurgence spike" — especially in Italy and Spain, where the incidence level exceeded pre-COVID figures. This indicates that collective immunity to the flu has "decreased" over two years without the circulation of the virus.
Genetic predisposition to influenza
Genetic susceptibility to the flu is not a myth, but a scientifically confirmed fact. Research published in the journal *Nature Genetics* (2021) identified several key genes that influence the severity of the disease:
— **IFITM3** (interferon-induced transmembrane protein 3): the rs12252-C mutation is associated with an increased risk of hospitalization. People with this variant have a defect in blocking the entry of the virus into the cell.
— **HLA-DQB1*06:02**: this variant of the major histocompatibility complex gene is associated with better recognition of viral peptides and a milder course of the disease.
— **MBL2** (mannose-binding lectin): polymorphisms of this gene reduce the effectiveness of innate immunity — such individuals are more likely to suffer severe illness.
Important: genetic predisposition is not a sentence. It only changes probabilities. For example, in a carrier of rs12252-C, the risk of severe influenza is 3–5 times higher, but if a person gets vaccinated annually and maintains hygiene — the risk remains low. In Spain, studies are being conducted on screening IFITM3 in patients with recurrent respiratory infections — this is not yet part of standard practice, but is already used in clinical trials.
Risk factors for flu occurrence
Risk factors can be divided into three groups: biological, behavioral, and environmental.
**Biological**:
— Age over 65 or under 2 years;
— Chronic diseases: COPD, asthma, coronary artery disease, type 2 diabetes, immunosuppression (e.g., due to HIV or after transplantation);
— Pregnancy (especially in the II–III trimester) — due to physiological suppression of immunity.
**Behavioral**:
— Refusal of vaccination;
— Smoking (damages the cilia of the respiratory epithelium);
— Sleep deprivation (less than 6 hours a day reduces interferon production);
— Poor nutrition — deficiency of vitamin D, zinc, selenium.
**Environmental**:
— Dense development (high-rise buildings with shared elevators and corridors);
— Work in "high-risk" professions: medical personnel, teachers, public transport drivers;
— Winter period — at a temperature of +5…+10°C and humidity 20–30% the virus remains viable for up to 48 hours on surfaces.
A special factor — **"office effect"**: in a closed room with air conditioning, where air circulates through common filters, the risk of infection increases 3 times compared to ventilated spaces. One study in Barcelona (2024) showed: in offices without regular replacement of HEPA filters, the level of viral particles in the air exceeded the permissible limit by 7 times.
Diagnosis of influenza: how to distinguish it from ARVI and other diseases
The main thing is not to confuse the flu with a common cold. Here are the key differences:
| Symptom | Flu | ARVI (for example, rhinovirus) |
|———|——-|——————————|
| Onset | Sudden (within 1–2 hours) | Gradual (1–2 days) |
| Temperature | High (38.5–40°C), lasts 3–5 days | Moderate (up to 38°C), 1–2 days |
| Muscle pain | Severe, "aching" | Mild or absent |
| Cough | Dry, deep, tormenting | Wet, weak |
| Nasal symptoms | Congestion late, scant mucus | Abundant rhinorrhea from the first day |
| Fatigue | Very pronounced, can last for weeks | Moderate, resolves in 2–3 days |
Diagnosis begins with clinical assessment but is confirmed in the laboratory:
— **Rapid antigen tests** (nasopharyngeal swab): result in 15 minutes, sensitivity 50–70% (better in the first 2 days of illness);
— **RT-PCR** — "gold standard": sensitivity >95%, allows determination of virus type (A/B) and subtype (H1N1, H3N2);
— **Serology** (ELISA for IgM/IgG): used retrospectively, during epidemiological investigation.
Radiological methods (X-ray, CT) are not needed for the diagnosis of influenza but are used when complications are suspected: pneumonia, pleuritis. X-ray in viral pneumonia shows diffuse foci, "ground glass," often bilateral.
Differential diagnosis includes:
— COVID-19 (symptoms are almost identical — PCR differentiation is needed);
— Adenovirus infection (often with conjunctivitis);
— Parainfluenza (characterized by a barking cough and laryngeal stenosis in children);
— Bacterial pneumonia (temperature does not decrease after 5 days, purulent sputum).
Treatment of influenza: what really helps and what does not
Treating influenza is not about "curing the virus," but about alleviating symptoms, preventing complications, and shortening the period of contagiousness (the time when you are infectious). Key principles:
**1. Antiviral therapy**
Drugs from the group of neuraminidase inhibitors (oseltamivir, zanamivir) are effective only when started within the first 48 hours of symptom onset. They reduce the duration of fever by 1–2 days and the risk of hospitalization by 60% in the elderly. In Spain, oseltamivir is available by prescription, but it is often in stock at pharmacies — especially in January–February.
**2. Symptomatic therapy**
— Antipyretics: paracetamol (first choice), ibuprofen — only if there is no stomach ulcer. Aspirin is prohibited for children under 16 (risk of Reye's syndrome).
— Cough suppressants: codeine — only by prescription, for dry cough that interferes with sleep. Do not give to children without a doctor's supervision.
— Runny nose: saline solutions (for example, "Aqua Maris") — safe and effective. Decongestants (xylometazoline) — no more than 5 days, otherwise the risk of mucosal atrophy.
**3. Immune support**
— Drinking regime: at least 2 liters of fluid per day (water, sugar-free compotes, green tea with lemon). Dehydration exacerbates intoxication.
— Bed rest: at least 5 days from the moment of temperature normalization. Even if "it’s already easier" — the body is still restoring resources.
— Vitamin D: if the level is below 20 ng/ml — prescribe 2000–4000 IU/day for 2 weeks. In Spain, vitamin D deficiency occurs in 40% adults in winter.
Surgical treatment for influenza is not used — except in extremely rare cases of lung abscess or pleural empyema requiring drainage.
List of medications used for influenza in Spain
Here is the current list of medications approved in Spain as of 2026, indicating the form of release and application features:
- Oseltamivir (Tamiflu) — capsules 75 mg Dose: 75 mg twice a day for 5 days. For prevention — 75 mg once a day for 7–10 days. Contraindicated in severe renal failure.
- Zanamivir (Relenza) — powder for inhalations. 2 doses (2×5 mg) twice a day for 5 days. Not recommended in case of bronchial asthma or COPD in the history.
- Baloxavir marboxil (Xofluza) — a new remedy, a single dose of 40 mg (children 5–12 years — 20 mg). Acts faster but is more expensive. Available in Spain since 2023, but requires PCR confirmation.
- Paracetamol (Panadol, Doloran) — tablets 500 mg. Up to 4 g/day for adults. Do not mix with other medications containing paracetamol.
- Ibuprofen (Nurofen) — 400 mg, 3 times a day. Only in the absence of peptic ulcer disease and with normal kidney function.
- Saline sprays (Salin, Aquamaris) — for nasal irrigation. Preservative-free — safe for pregnant women and children.
Important: antibiotics for influenza **are not prescribed**, if there are no signs of bacterial superinfection (purulent sputum, recurrent fever after subsiding, leukocytosis >15×10⁹/l). Self-prescribing antibiotics leads to resistance and dysbiosis.
Disease monitoring: control stages, prognosis, complications
After diagnosing influenza, it is important to organize monitoring. In Spain, for adults without risk factors, it is recommended:
— Temperature control 2 times a day for the first 3 days;
— Assessment of respiratory function: if shortness of breath worsens when climbing stairs — urgently to the doctor;
— Isolation period: at least 5 days from the onset of symptoms + 24 hours without fever (without antipyretics).
The prognosis is favorable for 95% patients with timely treatment. But there are risk groups where the prognosis is more serious:
— Elderly (>65 years): risk of pneumonia — 10–15%;
— Children with 39°C, respiratory rate >25 per minute, saturation <94%, altered consciousness, blood pressure <90 mm Hg — the emergency hospitalization protocol is automatically triggered.
Age-related features of influenza
Influenza in children is not a "little adult." In infants under 2 years, it often masquerades as a "cold," but with dangerous features:
— The temperature may be subfebrile (37.5–38.0°C), but the child is lethargic, refuses to breastfeed;
— Laryngotracheitis often develops — "false croup": sharp barking cough, stridor (wheezing breath), especially at night;
— The risk of seizures during fever is higher than with ARVI.
In adolescents and young adults, influenza occurs acutely but quickly — if there are no complications, recovery takes 5–7 days. However, it is in this group that "post-influenza asthenic conditions" are most often observed: weakness, dizziness, decreased concentration — lasting up to 2 months. This is not "laziness," but a real CNS recovery syndrome.
In elderly people (>65 years), influenza often "masquerades":
— The temperature can be normal or subfebrile;
— Main symptoms — worsening of well-being, confusion, drop in blood pressure;
— High risk of atypical pneumonia without cough;
— Mortality is 10 times higher than in young people.
In Spain, starting from 2024, the "Flu 65+" program is in effect: express tests and oseltamivir are provided for free at pharmacies when an elderly person presents with symptoms. This has reduced hospitalizations in Catalonia by 22% for the season.
Questions and answers: the most frequent patient inquiries
Question 1: Can you go outside with the flu if the temperature has dropped?
No. Even with a normal temperature, you remain contagious until the 7th day of the illness. The virus is released with droplets of saliva and mucus. If you need to go out — wear a medical mask, avoid public transport and crowded places. In Spain, violating quarantine (with a confirmed diagnosis) is punishable by a fine of up to 600 euros under law 33/2011 on public health.
Question 2: How long does the virus live on surfaces? And how to disinfect properly?
On metal and plastic — up to 48 hours, on paper and fabric — up to 8–12 hours. Effective disinfection:
— Surfaces: 0.1% sodium hypochlorite solution (regular bleach diluted 1:100) or 70% alcohol;
— Dishes: boiling for 5 minutes or dishwasher at 60°C+;
— Laundry: washing at 60°C, adding a chlorine-containing agent;
— Air: ventilate every 2 hours for 10 minutes, or HEPA filter in the room.
Question 3: If I got vaccinated but still got sick — did the vaccine not work?
Not necessarily. The vaccine does not guarantee 100% protection, but reduces severity by 60–80%. In 2025, the vaccine's effectiveness in Spain was 58% against the H3N2 strain and 72% against B/Victoria. This means you could get sick, but without pneumonia, without hospitalization — and that is already a victory for the vaccine.
Question 4: Can I breastfeed while having the flu?
Yes, and you should. Antibodies are passed to the child through milk. But mandatory measures:
— Wear a mask while feeding;
— Wash your hands before contact with the child;
— Disinfect the pump and bottles after each use;
— If it's hard, express milk to maintain lactation.
Question 5: When can I return to work after the flu?
Officially — after 5 days from the onset of symptoms + 24 hours without fever without antipyretics. But if the job involves children, the elderly, or the sick — it's better to wait 7 days. In Spain, employers are required to accept a sick leave certificate issued in form 060, even if you were treated on an outpatient basis.
Typical mistakes people make during influenza — and how to avoid them
Mistake #1: "I don't cough anymore — that means I'm recovered."
In fact, the virus continues to be shed until the 7th day. Even without a cough — when talking, sneezing, or blowing your nose, you infect those around you.
What to do: adhere to quarantine completely. Use separate dishes, towels, toothbrushes. Do not prepare food for the family for the first 5 days.
Mistake #2: "I'll take an antibiotic 'just in case'."
Antibiotics do not work on viruses. Taking them without indications leads to dysbiosis, allergies, and resistance.
What to do: take antivirals only as prescribed, with a confirmed diagnosis. If the temperature rises again after 5 days — then go to the doctor to look for a bacterial superinfection.
Mistake #3: "I'll ventilate the window — and it will be clean."
One ventilation is not enough. The virus remains on surfaces.
What to do: Combine: ventilation + disinfection + wearing a mask in the presence of others. Especially important in the bedroom and bathroom.
Mistake #4: "Children will get sick — and their immunity will strengthen."
This is a dangerous misconception. In children, the flu can lead to complications that do not occur in adults: myocarditis, Reye's syndrome, neurological consequences.
What to do: Vaccinate children from 6 months, especially if they attend daycare or school. In Spain, vaccination for children aged 6–24 months is free and mandatory as part of the national calendar.
Conclusion: how not to infect others — a step-by-step protection system
If you or your loved one has contracted the flu, do not panic, but act clearly. Here is a proven system that I recommend to my patients in Barcelona:
- The first 24 hours: take a PCR test (in Spain — in 30 minutes, 35 euros), start oseltamivir upon confirmation.
- Days 1–5: isolation in a separate room, mask when going into the hallway, daily disinfection of surfaces, separate dishes.
- Days 5–7: continue isolation even if you feel well. Check your temperature in the morning and evening.
- After 7 days: if there is no fever for 24 hours, you can go out — but wearing a mask, without contact with the elderly and children for another 2 days.
The main thing — remember: flu is not a personal problem, but a public health issue. In Spain, about 200,000 people become "invisible carriers" every year — they do not know they are sick and go to work, shops, and public transport. Your responsibility is to prevent this. Vaccination, hand hygiene, and wearing masks during the epidemic season are not fear, but respect for others. And yes — if you stayed home today, tomorrow you can return to life without guilt and with a clear conscience. Because you not only recovered — you preserved the health of those around you.
