Influenza
Overview
Influenza is an acute respiratory infectious disease caused by infection with influenza viruses (family Orthomyxoviridae). It causes seasonal epidemics every year and is estimated to be associated with hundreds of thousands of deaths worldwide. It is commonly called "flu," but it is a distinct disease from the common cold, differing in pathogen, severity, and prevention methods. In older adults, infants and young children, pregnant women, and people with chronic diseases, the risk of fatal complications such as pneumonia is high, so vaccination and early administration of antiviral agents are considered key management strategies.
Main content
Pathogen and classification
Influenza viruses are single-stranded RNA viruses, and subtypes are divided according to combinations of the surface antigens hemagglutinin (H) and neuraminidase (N). Seasonal epidemics in humans are mainly caused by influenza A (H1N1, H3N2) and B (Victoria and Yamagata lineages); type C causes mild sporadic infections, and type D is mainly a problem in livestock. Type A can undergo both antigenic drift and genetic recombination (antigenic shift), making it the main cause of pandemics.
Transmission routes
It is mainly transmitted through droplets produced by coughing and sneezing and through contact with contaminated surfaces, and airborne transmission is also considered possible in enclosed indoor settings. The incubation period is 1–4 days, and viral shedding is active before and after symptom onset, so asymptomatic and mildly infected people can also be sources of transmission. Explosive outbreaks occur in congregate settings such as schools, long-term care facilities, and military units.
Symptoms and course
Sudden high fever (38°C or higher), chills, muscle pain, headache, and fatigue are characteristic, accompanied by sore throat, dry cough, and runny nose. Most people recover within 1–2 weeks, but cough and fatigue may persist for several weeks. In children, gastrointestinal symptoms such as vomiting and diarrhea may be observed, and in older adults, fever may be less prominent.
Complications and high-risk groups
Secondary bacterial pneumonia (pneumococcus, Staphylococcus aureus, etc.), viral pneumonia, otitis media, sinusitis, myocarditis, encephalitis, and Guillain–Barré syndrome occur rarely. Adults aged 65 years or older, children under 5 years (especially under 2 years), pregnant women, people with chronic lung, cardiovascular, kidney, or liver disease, immunocompromised people, and people with obesity (BMI 30 or higher) are classified as high-risk groups.
Diagnosis
Clinical diagnosis is possible during epidemics, but confirmation is standardly made by reverse transcription polymerase chain reaction (RT-PCR). Rapid antigen tests are convenient but have low sensitivity, so a negative result cannot rule out infection. In severe patients or outbreak settings, viral culture and genetic sequencing are used to assess antiviral resistance.
Treatment
Neuraminidase inhibitors such as oseltamivir, zanamivir, and peramivir are used, and they are highly effective when administered within 48 hours of symptom onset. For high-risk groups and severe patients, administration should be considered regardless of the time of onset. In addition, supportive care such as rest, fluid intake, and antipyretics/analgesics is provided, and antibiotics are used only when secondary bacterial infection is confirmed.
Prevention and vaccines
The seasonal influenza vaccine, updated each year based on predictions of circulating strains, is the most effective prevention method. It is recommended for all people aged 6 months or older, with high-risk groups and healthcare workers as priority vaccination targets. Non-pharmaceutical interventions such as hand washing, cough etiquette, indoor ventilation, and refraining from going out when symptomatic are also important.
History and epidemiology
The 1918 Spanish flu (H1N1) is estimated to be associated with more than 50 million deaths worldwide, and there were pandemics in 1957 (Asian flu, H2N2), 1968 (Hong Kong flu, H3N2), and 2009 (novel influenza, H1N1pdm09). Avian influenza (H5N1, H7N9, H5N6, etc.) shows high fatality rates in sporadic human infections and remains under continuous surveillance.
Recent trends
In the 2024–2025 season, A(H1N1)pdm09, A(H3N2), and influenza B co-circulated, and influenza, which had been suppressed during the COVID-19 pandemic, rebounded above previous years' levels after relaxation of public health measures. In the United States, H5N1 highly pathogenic avian influenza infections in dairy cows and sporadic infections among dairy workers were reported, strengthening surveillance for zoonotic infections, and the Centers for Disease Control and Prevention (CDC) updated prevention guidance for occupational groups at risk of exposure. In the vaccine field, mRNA-based universal influenza vaccine candidates have entered clinical trials, and research on next-generation antivirals (such as baloxavir marboxil) and broadly neutralizing antibodies is also underway. In Korea, the Korea Disease Control and Prevention Agency monitors changes in circulating strains and antiviral resistance through sentinel surveillance and pathogen analysis, and operates a vaccination support program for high-risk groups.
Related topics
- [[Common cold]]
- [[COVID-19]]
- [[Vaccine]]
- [[Avian influenza]]
- [[Pneumonia]]