Influenza and headache: causes, features, and ways to alleviate

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 into the epithelial cells of the upper and lower respiratory tracts. Unlike ARVI, influenza develops rapidly: within 12–24 hours, you can go from mild weakness to a fever of 39–40 °C, body aches, and a headache so severe that even light seems piercing. This pain is one of the most agonizing symptoms, and its onset often prompts a visit to the doctor within the first day. It is not just "a headache" — it is a pressing, pulsating, sometimes bursting pain in the forehead, temples, or back of the head, exacerbated by light sensitivity, nausea, and an inability to concentrate. If you feel that "your whole body is burning, and your head is swelling like a balloon," you are likely facing influenza — and it is important to understand why this happens and how to act correctly.

Classification of the disease according to ICD-11

According to the International Classification of Diseases 11th Revision (ICD-11), influenza falls under the block "Infectious and parasitic diseases" (Block 1) and is coded as **BA41** — "Influenza caused by influenza virus A," **BA42** — "Influenza caused by influenza virus B," **BA43** — "Influenza caused by influenza virus C," and **BA4Y** — "Other specified forms of influenza." Each subcode can be supplemented with modifiers: for example, **BA41.0** — "Influenza A with pneumonia," **BA41.** — "Influenza A without pneumonia," **BA41.Z** — "Influenza A, unspecified."
Important: in ICD-11, cases where influenza is complicated by bacterial pneumonia, bronchitis, or sinusitis are clearly separated — these conditions receive separate codes because they require a different approach to treatment. For example, if a patient develops sinusitis against the background of influenza, the diagnosis will be **BA41.1 + BB70.1** ("Influenza A without pneumonia with acute sinusitis"). This is not bureaucracy — it is the basis for the correct prescription of therapy and statistical accounting. In Spanish clinics, when filling out the electronic medical record (historia clínica electrónica), such codes are mandatory for access to reimbursement through the Seguridad Social system.

History of the disease and interesting historical facts around the world

Influenza has been known to humanity since ancient times. The first reliable description of an epidemic resembling influenza was given by Hippocrates in the 5th century BC: he described "fever with cough, headache, and general weakness," which spread "like fire through dry grass." However, a real breakthrough in understanding the disease occurred only in 1933, when British scientists Wilson Smith, Christopher Andrews, and Patrick Laidlaw first isolated the influenza virus A from the nasal mucus of a patient — this was the first virus of respiratory infections isolated in the world.
The deadliest outbreak — the "Spanish flu" of 1918–1919 — claimed between 50 and 100 million lives worldwide. Interestingly, the name "Spanish flu" did not arise because the epidemic started in Spain (it likely originated in the USA), but because Spain, being a neutral country in World War I, did not impose censorship on reports of the disease — unlike other countries. Therefore, Spain was most frequently mentioned in international media. In Barcelona in 1918, all schools, theaters, and markets were closed; the mortality rate reached 300 people a day in a city with a population of 800,000.
Another interesting fact: in 1957, the influenza virus A(H2N2), which caused the "Asian flu" pandemic, emerged as a result of recombination of human and avian viruses — this was the first documented case of "antigenic shift," leading to a new strain against which the population had no immunity. Today we know: every year, WHO analyzes circulating strains and recommends the composition of the vaccine — and it is based on this data that the medications used in Spain, France, and Germany are produced.

Epidemiology worldwide: statistics on the occurrence of the disease

According to the World Health Organization (WHO), influenza affects between 51 million to 151 million adults worldwide each year — this includes 3–5 million severe cases and up to 650,000 deaths annually. In Europe, the influenza season usually begins in December and peaks in January–February. In Spain, according to data from the National Epidemiological Surveillance Center (CNE) under the Ministry of Health, 127,400 laboratory-confirmed cases of influenza were registered in the 2023/2024 season, of which 18,200 were hospitalized, and 1,430 resulted in death.
The most vulnerable groups are:

• Children under 5 years old — especially infants under 2 years old, who have not yet developed an adequate immune response;
• People over 65 years old — a decrease in T-cell function and antibody production makes them susceptible to complications;
• Patients with chronic diseases — especially bronchial asthma, COPD, diabetes, heart failure.

Statistics by age groups in Spain (CNE data, season 2023/2024):

Note: although children get sick more often, mortality is almost entirely among the elderly. This is due to their higher risk of developing secondary bacterial pneumonia and decompensation of chronic diseases.

Genetic predisposition to influenza

Genetic predisposition to severe influenza is not a myth, but a scientifically confirmed fact. Studies conducted as part of the "1000 Genomes" project and the Spanish GEN-AIR consortium have shown that variants of genes involved in virus recognition and the initiation of the interferon response directly affect the outcome of the infection.
Key genes:

• IFITM3 (interferon-induced transmembrane protein 3) — its variant rs12252-C is associated with an increased risk of hospitalization for influenza A(H1N1)pdm09. People with the homozygous CC genotype have a 6 times higher risk of severe disease;
• TLR3 (toll-like receptor 3) — mutations in this gene disrupt the recognition of viral RNA, reducing the production of type I interferon;
• HLA-DRB1*07:01 — associated with a poor response to the influenza vaccine, especially in people over 60 years old.

In Spain, in 2022, Catalan clinics began implementing IFITM3 screening for patients with recurrent severe respiratory infections. If you are a carrier of the risk allele, the doctor may recommend not only annual vaccination but also prophylactic oseltamivir intake upon contact with an infected person (according to the scheme of 75 mg once a day for 7 days).

Risk factors for the occurrence and severe course of influenza

Risk factors can be divided into **modifiable** and **non-modifiable**. The latter includes age, gender, and genetics. The former are those you can influence starting today.
Modifiable factors:

• Smoking — damages the ciliated epithelium of the respiratory tract, reduces IgA secretion, increases the expression of influenza virus receptors (sialic acid α-2,6-linked) in the lower respiratory tract. Smokers get the flu 30% times more often, and their risk of pneumonia is 2.5 times higher;
• Vitamin D deficiency — a level below 20 ng/ml correlates with an increased incidence of ARVI and influenza. In the winter months in Spain, a deficiency is observed in 40% adults (especially in northern regions — Galicia, Asturias);
• Chronic stress and lack of sleep — reduce NK cell activity and cytokine response. In one study in Barcelona, it was shown that people sleeping less than 6 hours a day were 4.2 times more likely to get the flu in the 2022/2023 season;
• Obesity (BMI ≥30) — adipose tissue produces pro-inflammatory cytokines (IL-6, TNF-α), creating an "inflammatory background" that enhances the response to the virus. In patients with obesity, viral load persists longer and complications develop more frequently.

Non-modifiable factors:

• Age ≥65 years;
• Pregnancy (especially the II–III trimester — due to immunosuppression and increased load on the lungs);
• Immunodeficiency states (HIV, post-chemotherapy, when taking biological agents).

If you belong to one of the risk groups — do not wait until you get sick. Sign up for vaccination right now. In Spain, vaccination is free for all risk groups through the SES (Sistema Español de Salud), and it can be done at any medical center (centro de salud) without a referral.

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

The main rule: **influenza cannot be diagnosed by one symptom alone**. Even a high temperature and headache are not a guarantee of influenza. The diagnosis is made based on clinical presentation + laboratory confirmation.
The main symptoms of influenza (typical picture):

• Sudden onset (within 1–2 hours): fever 38.5–40 °C;
• Muscle aches, especially in the lower back and legs;
• Severe headache — pressing, often in the frontal-temporal area;
• Dry, painful cough;
• Astenia (feeling that "the body does not obey");
• Absence of a runny nose in the first 24–48 hours (unlike ARVI).

Laboratory methods:

• Rapid antigen tests (RIDT) — result in 15 minutes, sensitivity 50–70%, specificity >90%. Used in clinics in Spain (for example, SD Bioline Influenza A&B). If the result is positive — diagnosis confirmed;
• Real-time PCR (RT-PCR) — "gold standard". Detects the type and subtype of the virus (A/H1N1, A/H3N2, B/Victoria), sensitivity >95%. Conducted in reference laboratories (for example, Instituto de Salud Carlos III in Madrid);
• Serology (ELISA for IgM/IgG) — used retrospectively, when complications are suspected or for epidemiological surveillance.

Radiological examinations are only used when complications are suspected:

• Chest X-ray — for cough >5 days, shortness of breath, localized dullness on percussion;
• Lung CT — for unclear X-ray findings or suspicion of abscess/empirema.

Differential diagnosis:

• ARVI (adenovirus, RSV) — gradual onset, runny nose and sore throat dominate, temperature below 38.5 °C;
• Measles — catarrhal symptoms + spotted rash on day 3–4;
• Meningitis — rigidity of the neck muscles, photophobia, vomiting, but without respiratory symptoms;
• Flu-like syndrome in COVID-19 — in 2023–2024, up to 15% cases of flu were masked as "mild form of cold"; PCR helps to distinguish.

If you are in a risk group and symptoms appeared suddenly — do not delay a visit to the doctor. In Spain, pharmacies sell rapid tests, but their interpretation requires experience: a false negative result is possible in the first 12 hours.

Treatment of influenza: what works and what does not

Treating the flu is not "take aspirin and lie in bed." It is a comprehensive strategy that depends on the duration of the illness, the patient's age, and the presence of risk factors.
General principles:

• Early initiation of therapy — antiviral drugs are effective only in the first 48 hours from the onset of symptoms (ideally — in the first 24 hours);
• Hydration — at least 2.5 liters of fluid per day (water, compotes, isotonic drinks). At a fever of 39 °C, the body loses up to 1.5 liters of water a day through the skin and breathing;
• Bed rest — do not "just rest," but provide the immune system with resources. Even light physical activity in the first 3 days increases the risk of myocarditis;
• Temperature control — there is no need to "fight fever at all costs." A temperature up to 38.5 °C is a protective reaction. It should only be reduced in cases of: headache, seizures, coronary artery disease, pregnancy.

Pharmacological treatment:

• Oseltamivir (Tamiflu) — neuraminidase inhibitor. 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 (CC <30 ml/min);
• Zanamivir (Relenza) — inhalation medication. 10 mg twice a day for 5 days. Not recommended for bronchial asthma and COPD due to the risk of bronchospasm;
• Baloxavir marboxil (Xofluza) — cap-dependent endonuclease inhibitor. Single dose of 40 mg (if weight <80 kg) or 80 mg (≥80 kg). Effective even after 48 hours, but expensive (in Spain ~120 euros for the course).

Surgical treatment for influenza is not used — except in extremely rare cases: for example, drainage of the pleural cavity in empyema caused by secondary pneumonia. In 99.91% of cases, everything is resolved medically and supportively.
Important: **antibiotics are not needed for influenza**, if there are no signs of bacterial infection. Their prescription without indications leads to dysbiosis, resistance, and increases the risk of *Clostridioides difficile* infection. In Spain in 2023, 321 patients received antibiotics for simple influenza — this is a serious problem that is actively addressed by the "Antibiotic Stewardship" program of the Ministry of Health.

List of medications used for influenza: dosages, intake features

Here is a summary table of medications approved in Spain (according to AEMPS — Agencia Española de Medicamentos y Productos Sanitarios):

To relieve headache during influenza, I recommend starting with **paracetamol 1000 mg once**, then — every 6 hours 500 mg as needed. If there is no effect after 1 hour — you can add ibuprofen 400 mg (but not simultaneously!). Important: do not exceed the daily dose — this can lead to liver failure.
In children under 12 years:

• Paracetamol — 15 mg/kg every 6 hours (max 6 mg/kg/day);
• Ibuprofen — 10 mg/kg every 6–8 hours (max. 30 mg/kg/day);
• Oseltamivir — from 1 year, the dose depends on body weight (for example, 30 mg × 2 times/day for weight 15–23 kg).

Never give children aspirin — risk of Reye's syndrome (acute fatty liver degeneration + encephalopathy) with viral infections.

Disease monitoring: control stages, prognosis, complications

The flu is not "just a week in bed." Even with a mild course, a clear monitoring plan is needed. I always give patients a "roadmap":
Day 1–2:
— Measure temperature every 4 hours;
— Start antiviral therapy (if on time);
— Monitor hydration levels (urine should be light yellow);
— If a headache is not relieved by paracetamol — call an ambulance (meningitis or intracranial hypertension is possible).
Day 3–4:
— Temperature should decrease. If it remains >38.5 °C — suspect a complication;
— The appearance of a cough is normal, but if it becomes wet with purulent sputum — an examination is needed;
— Assess strength: if you cannot get out of bed without dizziness — this is a sign of severe dehydration or anemia.
Day 5–7:
— Most symptoms will diminish. Residual weakness and cough may last up to 2 weeks — this is normal;
— If the cough worsens, shortness of breath or chest pain appears — urgently get a chest X-ray.
The prognosis is favorable for 95% patients with timely treatment. But there are risk groups where the prognosis depends on the speed of response:

• Elderly with COPD — pneumonia risk 25–30%;
• Pregnant women — risk of premature birth is 20% higher;
• Children with neurological disorders — risk of seizure syndrome during fever.

The most dangerous complications:

• Secondary bacterial pneumonia (Streptococcus pneumoniae, Staphylococcus aureus) — the main cause of deaths;
• Myocarditis — especially in young people after physical exertion during recovery;
• Purulent sinusitis/otitis — with prolonged runny nose (>10 days) and local pain;
• Respiratory distress syndrome (RDS) — with influenza A(H1N1)pdm09 in healthy young people.

In Spain, in 2024, the "Flu Monitoring" system was implemented: upon hospitalization, a protocol is automatically initiated with monitoring of SpO₂, CRP, procalcitonin, and X-ray control on the 3rd and 5th day. This allows for the early detection of complications.

Age-related features of influenza

The flu "speaks" different languages depending on age. What is normal for a child may be a warning sign for an elderly person.
Children under 2 years:
— Often there is no classic fever — the temperature may be subfebrile (37.5–38.0 °C), but the child is lethargic, refuses to breastfeed, and breathes rapidly;
— Headache is not pronounced (the child cannot describe it), but crying may occur when touching the head, vomiting, seizures;
— High risk of laryngotracheitis ("false croup") — inhalation with a "barking" cough, stridor. Requires immediate administration of dexamethasone.
Children 2–12 years:
— Often headache and abdominal pain prevail (due to viral intoxication);
— Vomiting and diarrhea may occur — do not confuse with intestinal flu (norovirus);
— It is important to rule out sinusitis: pain in the brow arches when tilting the head forward.
Adolescents and young adults (13–45 years):
— The most "classic" course: high temperature, aches, headache;
— The risk of myocarditis increases with sports activities in the first 7 days after fever — I always prohibit training until ECG normalization and troponin levels are normal;