When the flu turns into pneumonia: alarming symptoms and actions

Influenza is not just a "cold with a fever." It is an acute viral respiratory disease that can resolve in a week without consequences... or develop into severe pneumonia in just a few days. It is especially dangerous when a person underestimates the symptoms, delays seeking medical attention, or tries to "ride out" the illness at home with aspirin and lemon tea. In Spain, as in other European countries, the flu season brings up to 15-20 thousand hospitalizations annually, and about 3-5% of them are cases of bacterial pneumonia against the background of viral influenza. These are not numbers from a textbook: these are real people who started with a cough and weakness and ended up on a ventilator in intensive care. Today, I am Dr. Korzhikov, an infectious disease specialist from Barcelona, and I will tell you not only *when* influenza progresses to pneumonia but also *how* to recognize alarming signals at an early stage, which tests are truly necessary, and why "just an antibiotic" is not always the solution.

Classification of the disease according to ICD-11

According to the International Classification of Diseases 11th Revision (ICD-11), influenza is coded as BA41 - "Influenza caused by influenza virus A or B." Pneumonia that arises as a complication of influenza falls into the category CA20 - "Pneumonia caused by viruses," or CA21if a bacterial infection has joined (for example, Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus). Important: ICD-11 clearly distinguishes between primary viral pneumonia (rare but extremely severe form) and secondary bacterial pneumonia — it occurs in 90% of cases of complicated influenza in adults.
This distinction is not abstract. It affects treatment tactics: in viral pneumonia, antiviral therapy (oseltamivir, zanamivir) becomes key, while in bacterial pneumonia, the choice of antibiotic takes into account the local resistance profile. In Spanish clinics, for example, since 2020, the "CRB-65 + procalcitonin" algorithm has been actively used for differentiation: if procalcitonin > 0.25 ng/ml, the likelihood of bacterial infection increases threefold.

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 is found in the chronicles of Hippocrates (5th century BC), where he describes "fever with cough and shortness of breath spreading through the cities." But the real disaster occurred in 1918-1919 — the "Spanish flu" claimed 50 million lives worldwide. Interestingly, the name "Spanish flu" is a historical mistake. Spain was neutral in World War I, so it freely published data on the sick; in warring countries, censorship concealed the scale of the epidemic. In fact, the virus most likely originated in the USA (Kansas, spring 1918) and then spread through military units.
Another important fact: in 1957, the influenza A(H2N2) virus was first isolated, and in 1968 — A(H3N2). These strains formed the basis of modern vaccines. In Spain, in 1976, after an outbreak of swine flu in New Jersey, the first mass vaccination campaign began — however, due to a rare case of Guillain-Barré syndrome, the program was suspended. Today we know: the risk of this complication is 1 case per 1 million vaccinations, while the risk of hospitalization due to influenza is 1 in 1000. History teaches us: fear of vaccination is often more dangerous than the virus itself.

Epidemiology in the world (statistics of disease occurrence)

According to WHO, 3 to 5 million cases of severe influenza are registered worldwide annually, and 290-650 thousand deaths — predominantly among people over 65 years old, children under 5 years old, and individuals with chronic diseases. In Europe, the flu season lasts from October to April, with a peak in January-February. In Spain, according to the National Epidemiological Surveillance Center (CNE), the 2023/2024 season recorded:

• ≈ 1.2 million laboratory-confirmed cases of influenza;
• 14,800 hospitalizations with a diagnosis of "influenza";
• 1,840 cases of pneumonia as a complication of influenza;
• 312 deaths related to influenza and its complications.

Important: in 78% of cases of pneumonia against the background of influenza in adults, bacterial pathogens are identified. The most common is Streptococcus pneumoniae (45%), followed by Haemophilus influenzae (18 %) and methicillin-resistant Staphylococcus aureus (MRSA) - especially in patients with recent hospitalization or antibiotic therapy.
Table: Prevalence of pathogens in pneumonia against the background of influenza (according to ECDC, 2022–2025)

Note: viral pneumonia is the deadliest form because it develops rapidly and often requires mechanical ventilation by the 3rd–5th day of illness.

Genetic predisposition to this disease

Genetics plays a role, but not as a "sentence." Studies show that gene variants involved in the immune response can affect the severity of influenza and the risk of complications. For example:

• Gene IFITM3 (interferon-induced transmembrane protein 3): the rs12252-C mutation is associated with an increased risk of hospitalization for influenza A in young people. In Spain, this variation occurs in 2–3 % of the population - lower than in Asia (up to 25 %), but higher than in Scandinavia (0.5 %).
• Gene CCR5: the Δ32 deletion reduces the expression of the receptor used by some viruses to enter cells. Although this gene is more known in the context of HIV, there is evidence that it may partially protect against severe viral pneumonia.
• HLA-class II alleles: for example, HLA-DRB1*07 and HLA-DQB1*02 are associated with a weaker response to the influenza vaccine, which indirectly increases the risk of infection and complications.

However, genetics is just one element. Even with a "risky" mutation, a healthy lifestyle, timely vaccination, and adequate therapy minimize the risk. We cannot change genes, but we can control what depends on us.

Risk factors for the development of this disease

The risk of influenza progressing to pneumonia is not the same for everyone. There are factors that "prepare the ground" for bacterial colonization of the lungs:
Physical factors:

• Age over 65 or under 2 years — due to reduced immune reactivity;
• Chronic diseases: COPD, asthma, heart failure, diabetes (especially with HbA1c > 8 %);
• Immunosuppression: HIV with CD4 15 mg/day for more than 2 weeks;
• Smoking — damages the ciliated epithelium of the airways, reducing mechanical protection;
• Obesity (BMI ≥ 30) — associated with chronic inflammation and impaired alveolar function.

Chemical and behavioral factors:

• Late initiation of antiviral therapy (more than 48 hours after the onset of symptoms);
• Self-treatment with antibiotics without indications — creates resistance and disrupts the microbiome;
• Inadequate fluid intake — leads to thickening of mucus and stagnation in the bronchi;
• Hypothermia + physical exhaustion — temporarily suppresses local immunity in the nasopharynx.

If you have at least two of these factors — you are in a high-risk group. And yes, even if you "always get sick lightly" — this season may be different. Viruses mutate. Your body does not.

Diagnosis of this disease

Early diagnosis is your main shield. Do not wait until "the cough becomes dry and it hurts to breathe." Here’s what you need to know:
The main symptoms that distinguish pneumonia from simple influenza:

• Temperature > 38.5°C for more than 5 days (with influenza — usually 3–4 days);
• Cough worsening on the 5th–7th day, with purulent or rusty sputum;
• Shortness of breath at rest or with minimal exertion (for example, while talking);
• Chest pain that worsens with inhalation or coughing;
• Cyanosis (bluish discoloration of lips or nail beds);
• Confusion in the elderly — often the first sign of hypoxia.

Laboratory studies:

• Complete blood count: leukocytosis > 12×10⁹/l or leukopenia < 4×10⁹/l — a warning sign;
• C-reactive protein (CRP): > 100 mg/l indicates bacterial inflammation;
• Procalcitonin: > 0.5 ng/ml — high probability of bacterial pneumonia;
• PCR for influenza viruses (nasopharyngeal swab): the gold standard for confirming influenza;
• Sputum culture: performed in severe cases or therapy ineffectiveness — but results in 48–72 hours.

Radiological examinations:
Chest X-ray — a mandatory study when pneumonia is suspected. In influenza — normal or mild perivascular infiltration; in pneumonia — focal or segmental opacities, often in the lower lobes. In ambiguous cases — CT of the lungs (sensitivity 95% vs 70% for X-ray).
Differential diagnosis:
Not all "cough + fever" is pneumonia. It is necessary to exclude:

• Bronchitis (often dry cough, without focal changes on X-ray);
• Tuberculosis (slow development, night sweats, weight loss);
• Pulmonary embolism (sudden shortness of breath, chest pain, normal X-ray);
• Heart failure (leg swelling, orthopnea, heart enlargement on the image).

If you or your close one is at risk and at least 2 of the listed symptoms appear — do not delay visiting a doctor. In Spain, there is a system of "urgencias menores" — for such cases, you can make an appointment within 24 hours without a queue.

Treatment

Treatment should be comprehensive and start as early as possible. Remember: in suspected pneumonia, a 12-hour delay increases mortality by 8%.
General treatment:

• Bed rest until temperature normalizes and breathing stabilizes;
• Abundant drinking (2–2.5 l/day): water, compotes, herbal teas — but not alcohol and not coffee (they dehydrate);
• Oxygen therapy for SpO₂ < 94 %: at home — via nasal catheter (up to 2 l/min), in hospital — mask or HFNC;
• Expectorants (for wet cough): ambroxol, acetylcysteine — but not for dry cough!

Pharmacological treatment:

• Antiviral medications: oseltamivir (75 mg twice a day × 5 days) — effective if started within the first 48 hours. In the elderly and severe cases — the course can be extended to 10 days. Zanamivir — inhalational, but not in patients with bronchospasm.
• Antibiotics: prescribed when bacterial pneumonia is suspected. In outpatient practice in Spain, the first line is amoxicillin/clavulanate (875/125 mg twice a day) or levofloxacin (500 mg once a day). Upon hospitalization — combination: cefotaxime + azithromycin or moxifloxacin monotherapy.
• Glucocorticosteroids: only in severe pneumonia and hyperinflammation (CRP > 150 mg/l, FEV1 < 50 %). Prednisolone 0.5 mg/kg × 5 days — reduces the risk of mechanical ventilation by 30 %.

Surgical treatment:
Rarely, but it happens. Indications:

• Pleural empyema (pus in the pleural cavity) — drainage or video-assisted thoracoscopic decortication;
• Lung abscess > 4 cm, unresponsive to antibiotics — resection;
• Massive bleeding from the bronchus — bronchial embolization.

Important: surgery is not an alternative to therapy, but a complement in complications. Most cases are treated conservatively.

List of medications used to treat this disease

Here is the current list of medications recommended by the Ministry of Health of Spain (Ministerio de Sanidad, Guía Clínica de Neumonía 2024):

<td10 mg inhalational × 2 times/day × 5 days

Note: ibuprofen for influenza is a controversial issue. In 2019, WHO recommended avoiding NSAIDs in the first days, as they may exacerbate inflammation in the lungs. In Spain today, paracetamol is preferred as a safe option.

Disease monitoring

Treatment is not just taking pills. It is observation, correction, and prevention of relapses.
Control stages:

• Day 2–3: assessment of temperature, respiratory rate, SpO₂. If the temperature does not decrease — reconsider therapy;
• Day 5: repeat complete blood count and CRP. If CRP does not decrease by 50 % — resistance or complication may be possible;
• Day 7–10: control X-ray — to assess the resorption of infiltrates. Complete disappearance may take 4–6 weeks;
• In 4 weeks: spirometry in the presence of COPD or asthma — to identify post-infectious obstruction.

Forecast:

• With timely treatment — complete recovery in 85–90 % of cases;
• In the elderly (>75 years) with comorbidities — mortality risk 8–12 %;
• Recurrent episodes of pneumonia within a year — a reason for examination for immunodeficiency or tumor.

Complications:

• Empyema of the pleura (pus in the pleura) — requires drainage;
• Sepsis — in case of bacteremia, especially with MRSA;
• Acute respiratory distress syndrome (ARDS) — mechanical ventilation is needed;
• Post-pneumonic fibrosis — rare, but possible in severe viral form;
• Chronic respiratory failure — in patients with pre-existing COPD.

If you feel that you are "not getting better" — do not wait for the next appointment. Call emergency services (112 in Spain) or go to the nearest "urgencia."

Specific features of the disease

Influenza and pneumonia in children, adults, and the elderly — are almost different diseases.
Children under 5 years:

• Often starts with fever > 39°C, vomiting, refusal to eat;
• Pneumonia can occur without cough — only shortness of breath and "retraction" of the abdomen on inhalation;
• Risk — viral pneumonia from RSV or influenza B, less often — bacterial;
• Treatment: oseltamivir from 2 weeks, amoxicillin if bacteria are suspected. Antibiotics are not prescribed "just in case" — only as indicated.

Adults aged 18–65:

• Classic picture: chills → fever → cough → chest pain;
• Highest risk — among smokers, athletes (overexertion), people with diabetes;
• Important: in young people, pneumonia can develop rapidly — within 24 hours from "mild cough" to respiratory failure;
• Prevention: annual vaccination, hand washing, avoiding contact with sick individuals.

Elderly (>65 years):

• Often no fever — only weakness, confusion, decreased appetite;
• Pneumonia can be "lightning-fast" — with the development of sepsis within the first 48 hours;
• High risk of complications due to comorbidities (heart, kidneys, diabetes);
• Treatment: longer courses of antibiotics (10–14 days), mandatory monitoring of electrolytes and kidney function.

Remember: for an elderly person, "feeling unwell" is already a reason for examination. Don't say "he's just tired." It's better to check — and be reassured.

Questions and Answers

Question 1: Can pneumonia be treated at home without hospitalization?**
Yes, but only under strict conditions: age 18–65, absence of comorbidities, SpO₂ ≥ 94 % at rest, ability to take medications regularly, and access to a doctor within 24 hours. In Spain, outpatient treatment is allowed according to the CURB-65 scale: if the total score ≤ 1 — treatment at home is possible. Points: Confusion, Urea > 7 mmol/L, RR ≥ 30, BP < 90/60, Age ≥ 65. If 2 or more — hospitalization is mandatory.
Question 2: Why does the cough not go away for a month after the flu?**
This is post-viral bronchitis — a normal reaction. The virus damages the bronchial epithelium, and recovery takes 3–6 weeks. If the cough is dry, without fever and shortness of breath — it is not pneumonia. But if sputum with blood appears, chest pain or fever returns — urgently see a doctor. Sometimes an X-ray is needed to rule out residual infiltrate.
Question 3: Is vaccination against pneumococcus needed after pneumonia?**
Yes, it is mandatory. After pneumonia, immunity to pneumococcus weakens, and the risk of recurrence increases 3 times within a year. In Spain, it is recommended:
— Pneumovax 23 (23-valent) once;
— then Prevnar 20 (20-valent) after 1 year.
For individuals >65 years old — the scheme "Prevnar 20 → after 8 weeks Pneumovax 23". Vaccination reduces the risk of recurrent pneumonia by 45 %.
Question 4: Can the flu cause pneumonia without a fever?**
Yes, especially in the elderly and immunosuppressed. The so-called "atypical pneumonia" may manifest only as weakness, shortness of breath while walking, confusion, and low blood pressure. In such cases, the key marker is SpO₂ < 94 %. If a close person "just feels unwell," but has no fever — measure the blood oxygen. It can save a life.

Typical mistakes and how to avoid them

• Mistake: "I will take an antibiotic 'just in case'."
  Consequences: resistance, dysbiosis, masking of symptoms.
  How to avoid: antibiotics are prescribed only by a doctor, after evaluating laboratory data. In Spain, since 2020, the "antibiotic stewardship" system has been in place — every prescription is checked for justification.
• Mistake: "The fever has gone down — that means I have recovered."
  Consequences: hidden pneumonia progresses, and by day 7 — it is already a critical condition.
  How to avoid: continue treatment until the end of the course, even if you feel better. Monitor cough and shortness of breath — they are more important than fever.
• Mistake: "I am not vaccinated, but I am healthy — I am not afraid."
  Consequences: the risk of hospitalization for the unvaccinated is 4 times higher than for the vaccinated.
  How to avoid: vaccination is not a guarantee that you won't get sick, but a guarantee that you won't die. In Spain, vaccination is free for all risk groups and is recommended for everyone from 6 months.
• Mistake: "I am treating myself based on my neighbor's advice — she had the same thing."
  Consequences: inappropriate dosage, drug interaction, missed diagnosis.
  How to avoid: each case is unique. Even the same virus can manifest differently in different people. Consult a specialist.

Conclusion

The flu is not a "cold," but a potentially deadly infection, especially when it progresses to pneumonia. But it is important to understand: most cases are treatable if acted upon quickly and correctly. Key points that everyone should remember:

• Alarming symptoms are not "just another day with a cough," but shortness of breath at rest, fever > 38.5°C for more than 5 days, purulent sputum, confusion;
• Diagnosis should include X-ray and procalcitonin — do not skimp on examination;
• Treatment is comprehensive: antivirals + antibiotics (if indicated) + oxygen + rest;
• Prevention — vaccination, hygiene, healthy lifestyle — works better than any medicine.

Don't be the hero who "overcomes it". Be wise — and take care of yourself. Because your life is not a statistic. It is yours.