InfluenzaEdit

Influenza, commonly called the flu, is an acute viral illness that primarily targets the respiratory tract. It is spread through droplets and, in severe cases, can lead to pneumonia and other complications. Each year, seasonal influenza causes substantial illness worldwide, with waves that typically peak during the winter months in temperate regions. The disease burden includes hospitalizations, lost productivity, and mortality, especially among older adults, young children, pregnant people, and those with chronic health conditions. Beyond regular seasonal activity, influenza has produced several pandemics when a novel virus emerged to which the global population had little preexisting immunity.

Influenza viruses belong to the family Orthomyxoviridae. The principal human pathogens are Influenza A virus and Influenza B virus, with Influenza C virus generally causing milder disease. Influenza A and B circulate seasonally in humans, while influenza C contributes less to annual illness. Influenza viruses are further characterized by surface proteins; influenza A subtypes such as H1N1 and H3N2 are frequently involved in human outbreaks. The viruses are subject to antigenic drift—small, continuous changes in their surface proteins—and, on occasion, antigenic shift, which can generate novel strains with pandemic potential. Global surveillance tracks circulating strains to inform vaccine design and other public health responses. Global influenza surveillance and response system networks, coordinated by organizations like World Health Organization, play a central role in this effort.

Virology

Influenza viruses are enveloped, negative-sense RNA viruses with segmented genomes. The major human pathogens are influenza A, B, and C, with A and B responsible for most clinical illness. Subtypes of influenza A are defined by their hemagglutinin (H) and neuraminidase (N) surface proteins; common subtypes include H1N1 and H3N2. The genetic plasticity of these viruses underpins seasonal variability and, occasionally, pandemic emergence. For background on the viral families and proteins, see Hemagglutinin and Neuraminidase.

Epidemiology

Seasonal influenza circulates worldwide, with higher activity in the colder months of temperate regions and more year-round activity in tropical areas. Population-level immunity against circulating strains, vaccination coverage, and viral fitness all influence the size and impact of seasonal waves. Certain groups—such as the elderly, infants, pregnant people, and those with chronic medical conditions—face higher risks of complications and death. History has shown that influenza can occasionally cause pandemics when a new strain, to which most people have little immunity, emerges—most notably in the 1918 influenza pandemic, the [1957 influenza pandemic], the [1968 influenza pandemic], and the 2009 [H1N1 pandemic|H1N1] event. For more on past pandemics, see pandemic influenza.

Transmission and clinical features

Influenza spreads primarily through respiratory droplets produced when infected people cough, sneeze, or talk. It can also spread via contact with contaminated surfaces, though this is typically a secondary route. The incubation period is short, usually one to four days. Most people experience abrupt onset of fever, cough, sore throat, myalgias, and fatigue, but presentations can vary. While many recover within a week, influenza can lead to serious complications, including pneumonia, particularly in the very young, the elderly, and people with underlying health conditions. More severe strains or coexisting illnesses can drive hospitalizations and strain health systems. For broader context on how communities monitor and respond, see public health efforts surrounding seasonal outbreaks and pandemics.

Diagnosis and treatment

Diagnosis combines clinical assessment with laboratory testing, including rapid antigen tests and molecular assays such as RT-PCR, which can distinguish influenza from other respiratory pathogens. Antiviral medications, when given early, can reduce symptom duration and the risk of complications. The main classes are neuraminidase inhibitors such as oseltamivir and zanamivir, which interfere with viral release. Resistance to antivirals can emerge, so treatment decisions consider local resistance patterns, patient risk factors, and disease severity. Supportive care remains important, and vaccination is the foundational strategy to reduce incidence and severity.

Prevention and public health

The primary preventive measure is vaccination. [Influenza vaccines] are reformulated annually to match circulating strains, guided by input from global monitoring networks and national health authorities. Vaccine composition typically targets the most prevalent influenza A and B strains and has become a central element of public health strategy in many countries. Beyond vaccination, public health measures such as hand hygiene, respiratory etiquette, and, during severe seasons, targeted vaccination campaigns for high-risk groups, contribute to reducing transmission. Discussions about vaccination policies—such as mandates for healthcare workers or school settings—exist as part of broader debates on public health, individual liberty, and the allocation of limited healthcare resources. See related discussions in vaccine mandate and health policy discourse for a balanced view of competing priorities and practical considerations.

Economic and policy considerations

Influenza imposes a recurring burden on health systems and economies through hospital care, outpatient visits, and productivity losses. Vaccine uptake, manufacturing capacity, distribution logistics, and the timing of vaccine campaigns influence the overall effectiveness of preventive efforts. Policymakers weigh the costs and benefits of various interventions, balancing population health gains against concerns about cost, access, and personal choice. This landscape includes ongoing deliberations about funding for surveillance programs, incentivizing vaccine development, and prioritizing protection for high-risk populations while ensuring broad access.