Influenza (Seasonal Flu)

Infectious Disease

Description

Seasonal influenza is an acute, highly contagious respiratory illness produced by orthomyxoviruses—chiefly influenza A and B—that circulate worldwide in annual epidemics and cause sudden fever, cough, myalgia, headache, and malaise. In temperate zones—such as North America and Europe—epidemics erupt during winter; in the tropics activity may persist year-round with semi-annual peaks. Worldwide, influenza causes up to 650 000 respiratory deaths each year, and in the 2024-2025 northern-hemisphere season A(H1N1)pdm09 has predominated with later co-circulation of B/Victoria lineage viruses.

Symptoms

After a one- to four-day incubation, illness strikes abruptly with high fever, chills, dry cough, retro-orbital headache, diffuse myalgia—classically “bone-breaking” pain—profound fatigue, sore throat, and rhinorrhea. Gastro-intestinal upset occurs in children. Most immunocompetent adults improve within five to seven days, but cough and tiredness can linger for weeks. In older or high-risk persons, primary viral pneumonia or secondary bacterial superinfection may manifest with dyspnea, pleuritic pain, purulent sputum, or sepsis.

Risk Factors

Extremes of age, pregnancy, chronic cardiopulmonary, metabolic, renal, hepatic, or neurologic disorders, immunosuppression, obesity with body-mass index ≥ 40 kg m⁻², residence in long-term-care facilities, and health-care or daycare exposure increase the likelihood of infection, severe disease, and complications. Lack of annual vaccination and cigarette smoking further heighten risk.

Diagnosis

Clinical recognition during a local outbreak is often sufficient in outpatients. Confirmation relies on nucleic-acid amplification tests—rapid RT-PCR or multiplex respiratory panels—that detect and subtype influenza in nasopharyngeal swabs within hours; these guide antiviral and cohort decisions. Antigen tests are less sensitive. Chest imaging evaluates suspected pneumonia. Differential diagnoses include SARS-CoV-2, respiratory syncytial virus, adenovirus, Mycoplasma pneumoniae, and allergic rhinitis.

Treatment

Otherwise healthy outpatients within forty-eight hours of symptom onset benefit from oral oseltamivir 75 mg twice daily for five days, which abbreviates illness by one to two days and reduces complications. Inhaled zanamivir, single-dose baloxavir, and intravenous peramivir are alternatives based on age and comorbidities. Antivirals are indicated at any time in those hospitalized, pregnant, severely ill, or with chronic conditions. Supportive care includes rest, hydration, antipyretics, and cough relief. Broad-spectrum antibiotics target secondary bacterial pneumonia when clinically suspected. Corticosteroids are not recommended unless another indication exists.

Outlook

The vast majority recover fully. High-risk groups, however, sustain hospital-admission rates of 50–100 per 100 000, and global case-fatality averages 0.1 % but climbs to 5 % in ICU cohorts. Annual vaccination halves hospitalizations and, in 2024-2025 U.S. interim analyses, cut outpatient visits by roughly one-third and admissions by a similar magnitude.

Complications

Secondary bacterial pneumonia (often Streptococcus pneumoniae, Staphylococcus aureus), primary viral pneumonitis, acute respiratory distress syndrome, myocarditis, pericarditis, encephalitis, Guillain-Barré syndrome, febrile seizures, myositis or rhabdomyolysis, decompensation of chronic diseases, and in pregnancy pre-term labor and fetal distress are recognized sequelae. Post-influenza inflammatory multi-system syndromes remain rare.

Prevention

Annual intramuscular inactivated or recombinant influenza vaccine is recommended for everyone aged six months and older. For 2024-2025 the trivalent formulation includes updated A(H1N1)pdm09, A(H3N2), and B/Victoria antigens; WHO has already issued seed-strain recommendations for 2025-2026. Vaccinate ideally in September or October but continue while viruses circulate. Standard precautions—hand hygiene, face masks in crowded indoor settings, and staying home when ill—further curb transmission.

Support

Isolate at home until at least twenty-four hours after fever abates without antipyretics; cover coughs, discard tissues promptly, and wash hands or use 60 % alcohol gel. Maintain adequate fluids, use acetaminophen or ibuprofen for fever and aches, and avoid salicylates in children. Adhere to the full antiviral course even if symptoms improve early. Household members at high risk should contact clinicians about post-exposure prophylaxis with oseltamivir.