Flu and temperature: when fever is a signal of alarm, and when it is normal

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 can mutate at a frightening speed. Unlike ARVI, influenza begins suddenly: in just a few hours, you can go from mild fatigue to a fever of 40 °C, body aches, and an inability to get out of bed. The fever in influenza is not a side effect, but part of the immune response. However, it is important to understand here: a high temperature is not always dangerous in itself — it becomes a signal of alarm when the balance between the body's defense and its own forces is disrupted. Many people, especially parents of small children or elderly patients, panic at the first rise of the thermometer, hastily lowering the fever without considering that this "fever" helps fight the virus. Others, on the contrary, ignore the temperature until complications arise — pneumonia, myocarditis, exacerbation of chronic diseases. Today we will discuss how to recognize when a fever is a normal immune response and when it is a sign that the body is no longer coping. And most importantly — how to act without panic, but with a clear plan.

Classification of the disease according to ICD

According to the International Classification of Diseases 11th Revision (ICD-11), influenza falls under the category "Infectious and parasitic diseases" (block 1A), subcategory "Viral infections of the respiratory tract." The code for the main diagnosis is **BA40** — "Influenza caused by influenza virus A or B." There are also specific codes:
— **BA40.0** — Influenza caused by influenza virus A;
— **BA40.1** — Influenza caused by influenza virus B;
— **BA40.2** — Influenza caused by influenza virus C (rarely causes severe forms);
— **BA40.Y** — Other specified forms of influenza;
— **BA40.Z** — Influenza, unspecified.
Important: if influenza is complicated by pneumonia, the code changes to **BA41** — "Influenza with pneumonia," and in the case of bacterial superinfection, an additional code is added (for example, **BA52** for bacterial pneumonia). This is not a bureaucratic detail — correct coding affects treatment assignment, hospitalization, and even epidemiological surveillance. For example, a case of influenza with code BA41 automatically enters the monitoring system of Rospotrebnadzor as potentially dangerous to public health.

History of the disease and interesting historical facts

Influenza has been known to humanity since ancient times. The first reliable description of an epidemic resembling influenza appears in the works of Hippocrates (5th century BC) — he described "fever with cough and chest pain," which spread "like fire through dry grass." However, a real breakthrough in understanding influenza occurred 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. Interestingly, the name "Spanish flu" arose not because the virus originated in Spain (it was indeed first officially registered there), but because Spain, being a neutral country in World War I, did not censor news about the disease — unlike other countries. Thus, the world learned about the scale of the disaster.
Another important fact: in 1933, British scientists Wilson Smith, Christopher Andrews, and Patrick Laidlaw first isolated the influenza A virus from the nasal mucus of an infected patient. This marked the beginning of the era of virology. In the 1940s, after World War II, the first vaccines against influenza were created — initially for the military, then for the civilian population.
Today we know that influenza is not a single disease, but a whole "family clan" of viruses. Influenza A viruses are divided into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). For example, H1N1, H3N2, H5N1 — these are different "faces" of one virus. It was H5N1 ("bird flu") that caused panic in 2005–2006 when the virus transmitted from birds to humans with a mortality rate of up to 60%. But fortunately, it did not learn to be effectively transmitted from person to person — otherwise, the consequences would have been catastrophic.

Epidemiology (statistics of disease occurrence)

According to WHO, annually, influenza affects from 51 million to 101 million adults worldwide and from 201 million to 301 million children. This amounts to about **1 billion cases of ARVI**, of which 3–5 million are severe forms requiring hospitalization. Each year, between 290,000 and 650,000 people die from influenza — predominantly elderly people, children under 5 years old, and patients with chronic diseases.
In Russia, the flu season usually begins in late October — early November and peaks in January–February. According to statistics from Rospotrebnadzor over the past 5 years:
— The average incidence rate is 150–250 cases per 10,000 population during the epidemic season;
— In years of "strong" strains (for example, 2019/2020 — H1N1pdm09, 2022/2023 — H3N2), the figures jumped to 400–500 per 10,000;
— The highest incidence is among children aged 1–4 years (up to 600 per 10,000), followed by schoolchildren aged 5–14 years;
— Among individuals over 65 years old, the incidence is lower (about 100–150 per 10,000), but the mortality rate is higher — up to 15–20% among hospitalized patients.
Important: statistics do not account for "hidden" cases — when a person has had a mild form at home without seeking medical attention. Studies using serological analysis show that the actual coverage of influenza may be 2–3 times higher than official figures.

Genetic predisposition to this disease

Genetic predisposition to severe influenza is not a myth, but a scientifically confirmed fact. Research conducted as part of the "1000 Genomes" project and based on biobanks (for example, UK Biobank) has identified several key genes that influence susceptibility to the virus and the severity of symptoms.
The most studied gene is **IFITM3** (interferon-induced transmembrane protein 3). It encodes a protein that blocks the virus from entering the cell at an early stage. In people with the rs12252-C mutation in this gene, the risk of developing severe influenza increases by 6 times. This mutation occurs in 25% of the Asian population and only in 4% of Europeans — which partially explains the differences in the epidemiological picture.
Other genes:
— **TLR7** — is responsible for recognizing viral RNA. Mutations here are associated with increased sensitivity to influenza in men (the gene is located on the X chromosome);
— **HLA-DQA1*01:02** — a variant of the major histocompatibility complex gene that reduces the effectiveness of the T-cell response;
— **MBL2** — encodes mannose-binding lectin, which is involved in innate immunity. A deficiency of this protein increases the risk of bacterial complications.
This does not mean that a "bad gene" condemns you to severe influenza. Genetics is just one of the factors. But if your family often experiences complications after influenza, it is worth paying attention to prevention: vaccination, hygiene, timely treatment.

Risk factors for the development of this disease

Risk factors can be divided into three groups: biological, behavioral, and environmental.
**Biological factors:**
— Age: children under 5 years and people over 65 have a reduced immune response;
— Chronic diseases: bronchial asthma, COPD, diabetes, heart failure, immunodeficiencies (including HIV and post-chemotherapy conditions);
— Pregnancy — especially the II and III trimesters: hormonal changes and physiological immunosuppression increase the risk of complications by 4–5 times;
— Obesity (BMI ≥30) — adipocytes secrete pro-inflammatory cytokines, exacerbating the "cytokine storm" reaction.
**Behavioral factors:**
— Smoking — damages the cilia of the respiratory epithelium, reducing their ability to expel the virus;
— Lack of sleep and chronic stress — suppress the production of interferons;
— Refusal of vaccination — the most significant modifiable risk factor.
**Environmental factors:**
— Closed spaces with poor ventilation (schools, offices, transport);
— High population density;
— Seasonality — cold and low humidity (less than 40% RH) increase the stability of the virus in the external environment and reduce the protective functions of the nasal mucosa.
Special attention — to people working in service, healthcare, and education. Their risk of infection is 2–3 times higher than average. If you belong to one of these groups — do not wait until you get sick. Act in advance: get vaccinated, use masks during peak season, wash your hands every 2 hours.

Diagnosis of this disease

Diagnosis of influenza is a combination of clinical picture, rapid tests, and laboratory methods. The main thing is not to confuse it with ARVI or coronavirus infection, as the treatment and prevention tactics differ.
**Main symptoms of influenza:**
— Sudden onset (within 1–2 hours);
— Temperature ≥38.5 °C (often 39–40 °C);
— Intoxication: headache, muscle and joint pain, weakness, photophobia;
— Respiratory symptoms: dry cough, nasal congestion, sore throat — but they appear later than fever;
— In children — possible vomiting, diarrhea, seizures at high temperature.
**Laboratory studies:**
— Rapid tests for influenza virus antigen (nasal/throat swab): result in 15–30 minutes, sensitivity 50–70%. A positive result is a reason to prescribe antiviral medications.
— PCR diagnostics: gold standard. Detects viral RNA, determines subtype (A/B, H1/H3, etc.). Sensitivity >95%, but requires 4–24 hours.
— Serological method (ELISA for antibodies): used for retrospective diagnosis (10–14 days after the onset of illness), when IgM and IgG appear in the blood.
**Radiological examinations** are only needed when complications are suspected:
— Chest X-ray — in case of cough, shortness of breath, persistent fever >5 days;
— CT of the lungs — in case of suspicion of atypical pneumonia or abscess.
**Differential diagnosis** includes:
— ARVI (gradual onset, temperature ≤38.5 °C, predominance of runny nose and cough);
— COVID-19 (often — loss of smell, more pronounced shortness of breath, prolonged fever);
— Parainfluenza, RSV infection (more common in children, with a "barking" cough and laryngeal stenosis);
— Bacterial pneumonia (temperature "constant", does not decrease after antipyretics, purulent sputum).
If you are unsure — do not self-diagnose. Consult a therapist or infectious disease specialist. Especially if the temperature lasts more than 3 days or there are signs of deterioration: shortness of breath, chest pain, confusion.

Treatment

Treatment of influenza is not a direct fight against the virus, but support for the body to cope on its own. Antiviral medications work only in the first 48 hours from the onset of symptoms. After that, their effectiveness drops sharply.
**General treatment:**
— Bed rest for at least 5–7 days (even if the temperature has dropped — the virus is still active);
— Abundant warm drinks (at least 2 liters/day): compotes, fruit drinks, green tea with lemon. Avoid coffee and alcohol — they dehydrate;
— Ventilation of the room every 2 hours for 10–15 minutes;
— Humidification of the air to 50–60% — dry air irritates the mucous membrane and slows down recovery.
**Pharmacological treatment:**
— Antiviral agents: **oseltamivir** (Tamiflu), **zanamivir** (Relenza), **baloxavir marboxil** (Xofluza). They inhibit neuraminidase or capsid polymerase, preventing the virus from exiting the cell.
— Antipyretics: **paracetamol** (Panadol, Efferalgan) — first choice. **Ibuprofen** is permissible, but with caution in children and in cases of peptic ulcer disease.
— Cough suppressants and expectorants — only by doctor's prescription. For dry cough — codeine or butamirate; for wet cough — ambroxol, acetylcysteine.
**Surgical treatment** for influenza is not used — except in rare cases: for example, in the case of lung abscess or empyema of the pleura, which require drainage. But this is already a complication, not the disease itself.
**Other types of treatment:**
— Inhalations with saline or mineral water (Borjomi, Narzan) — moisturize the mucous membrane;
— Physiotherapy (UHF, magnetotherapy) — only after the temperature has subsided and on the recommendation of an ENT specialist;
— Immunomodulators (for example, ribomunil, cycloferon) — their effectiveness is questionable, and they do not replace vaccination.
The main rule: do not take antibiotics "just in case." Influenza is a viral infection. Antibiotics do not work on viruses and can cause dysbiosis, allergies, or bacterial resistance.

List of medications used to treat this disease

Below is the current list of medications approved for use in the Russian Federation as of 2026. All of them must be prescribed by a doctor, especially for children and pregnant women.

Important: many "folk" remedies (onion, garlic, honey with milk) do not treat influenza but can alleviate symptoms. For example, honey reduces the frequency of nighttime cough in children over 1 year old (according to Cochrane Review 2022). But it does not replace antipyretics at a temperature of 39 °C.

Disease monitoring

Monitoring influenza is not just "looking at the thermometer." It is a systematic assessment of dynamics to timely notice the transition from a simple infection to a complication.
**Control stages:**
— Day 1–2: assessment of temperature, well-being, taking antiviral (if prescribed);
— Day 3–4: if the temperature does not decrease below 38 °C — the diagnosis or prescription needs to be reconsidered;
— Day 5–7: assessment of respiratory symptoms — cough should become wet, phlegm should be clear or white;
— Day 8+: if the temperature has risen again or shortness of breath has appeared — see a doctor immediately.
**Prognosis** with timely treatment is favorable: recovery occurs within 7–10 days. But for at-risk groups, the prognosis may worsen:
— In the elderly — the risk of pneumonia 10–15%;
— In pregnant women — premature birth, intrauterine hypoxia;
— In children — convulsive syndrome, encephalopathy.
**Complications** are divided into:
— Pulmonary: viral pneumonia, bacterial superinfection (streptococcus, staphylococcus), bronchiolitis;
— Extrapulmonary: myocarditis, pericarditis, meningitis, sinusitis, otitis;
— Systemic: cytokine storm, acute respiratory distress syndrome (ARDS), multiple organ failure.
If you or your child experience any of these conditions — do not delay visiting a doctor. Complications develop quickly: from the first deterioration to a critical condition can take only 12–24 hours.

Age-related features of the disease

Influenza in children, adults, and the elderly is almost different diseases. The difference is not in the virus, but in the immune system's response.
**In children under 3 years:**
— Often there is no classic fever — the temperature can be 37.5–38.5 °C, but the child is lethargic and refuses to eat;
— High risk of seizures at temperatures >38.5 °C (febrile seizures);
— Otitis media and laryngitis ("false croup") often accompany;
— Important: do not give aspirin — risk of Reye's syndrome (acute liver failure).
**In children 3–14 years:**
— Classic picture: sharp rise in temperature, body aches, headache;
— Often — vomiting and diarrhea (especially with H1N1);
— Schoolchildren are "super spreaders": infect up to 10 people a day.
**In adults aged 18–60:**
— The most "predictable" age: pronounced intoxication, but quick recovery with treatment;
— The risk of complications increases with the presence of chronic diseases (asthma, diabetes);
— In men, severe course is more common (due to the peculiarities of the immune response).
**In individuals over 65 years old:**
— Temperature may be "masked" — 37.2–37.8 °C, but the condition is severe: confusion, shortness of breath, tachycardia;
— High risk of pneumonia (up to 30% of hospitalized);
— Often — exacerbation of coronary artery disease, chronic heart failure, COPD;
— Mortality from untreated influenza — up to 15%.
If you are caring for an elderly person — do not wait for the temperature to rise. Pay attention to the general condition: lethargy, refusal to eat, dry mouth, rapid breathing. These signs may indicate a hidden infection.

Questions and Answers

Question 1: Is a temperature of 39.5 °C in an adult dangerous? Should it be brought down immediately?
No, not necessarily. A temperature up to 40 °C is a normal immune system response. Interferons and leukocytes work more effectively at 38.5–39.5 °C. Bring down the fever if:
— Temperature ≥40 °C;
— There is a headache, nausea, confusion;
— The patient has cardiovascular diseases;
— The temperature lasts more than 3 days without decrease.
First dose — paracetamol 500–1000 mg. Do not combine with ibuprofen without a doctor's prescription.
Question 2: Is it possible to go outside with a temperature of 37.5 °C and a cough?
No. Even with a subfebrile temperature, you remain contagious for 5–7 days. The virus is released with droplets of saliva and mucus. Going outside not only risks your health (hypothermia will worsen inflammation) but also poses a threat to others — especially the elderly and children. Stay home for at least 5 days from the moment the temperature normalizes.
Question 3: Why does a cough persist for 2–3 weeks after the flu?
This is not a "residual virus," but a consequence of damage to the respiratory mucosa. The virus destroys the cilia of the epithelium, and recovery takes time. Bronchial hyperreactivity is also possible (as in asthma). If the cough is dry and lasts longer than 3 weeks — a consultation with an ENT or pulmonologist is needed to rule out chronic bronchitis or an allergic reaction.
Question 4: The vaccination didn't help — I got sick a week later. Why then get vaccinated?
Vaccination does not guarantee 100% protection, but it reduces the severity of the disease. Studies show: among vaccinated people, the risk of hospitalization decreases by 40–60%, and mortality by 70–80%. Even if you get sick, you are likely to have experienced the flu more mildly than you could have. Moreover, the vaccine protects against several strains at once — and if one "slipped through," others were blocked.
Question 5: Can antibiotics be taken "preventively" against complications?
No. Antibiotics do not act on viruses and do not prevent bacterial complications. On the contrary, their uncontrolled use leads to dysbiosis, allergies, and resistance. Antibiotics are prescribed only for confirmed bacterial infections (for example, purulent sputum, leukocytosis, radiological signs of pneumonia). Prevention of complications includes vaccination, hygiene, and timely treatment of the flu.

Typical mistakes and how to avoid them

1. Mistake: "I will lower the temperature — and I will immediately feel better."
Reducing fever does not speed up recovery — it only alleviates discomfort. Often after taking an antipyretic, a person gets up, goes to work, gets chilled — and the illness worsens.
How to avoid: Lower the temperature only when it exceeds 38.5 °C or in case of poor tolerance. Even after lowering it — stay in bed for at least 2 days.
2. Mistake: "I have had a fever for 3 days — I urgently need antibiotics."
The first 3 days is normal for a viral infection. Antibiotics will not help, but may cause diarrhea or allergies.
How to avoid: Wait until the 5th day. If the fever has not subsided, purulent sputum or shortness of breath has appeared — then see a doctor for an antibiotic prescription.
3. Mistake: "I took Tamiflu for 2 days — and stopped because I felt better."
An untreated antiviral course leads to relapse and virus resistance.
How to avoid: Take the medication strictly according to the scheme — 5 days, even if your condition improves on the 2nd day.
4. Mistake: "The child doesn't want to drink — let them lie down."
Dehydration in children develops within 6–12 hours. It worsens intoxication and increases the risk of seizures.
How to avoid: Give small sips every 15 minutes: compote, juice, rehydron. If the child refuses — use a syringe without a needle (2–3 ml at a time).
5. Mistake: "I had the flu — now I have immunity for a year."
Immunity to a specific strain lasts 1–2 years, but the virus mutates every year. A new subtype may appear in the next season.
How to avoid: Get vaccinated every year — even if you have had it. The vaccine is updated for current strains.

Conclusion

The flu is not "just a cold," but it is not a death sentence either. The key to safe recovery is understanding when the fever works for you and when it works against you. A high fever in the first 2–3 days is a signal that the immune system is fighting. But if it does not subside, is accompanied by shortness of breath, chest pain, or confusion — this is no longer a fight, but a cry for help.
Do not fear the fever, but do not ignore it. Do not skimp on vaccination — it is 10 times cheaper than hospitalization. Do not treat "by analogy" with last year — each flu season is unique. And most importantly: if you are unsure — consult a doctor. It is better to spend 30 minutes on a consultation than a week in the hospital.
Remember: your task is not to "kill the virus," but to give the body a chance to defeat it on its own. And for that, you need rest, water, time, and a sensible approach. I am Dr. Korzhikov, and I want you to experience the flu not as a disaster, but as a trial that you have passed with wisdom and care for yourself.