Influenza viruses are enveloped, segmented, negative-sense, single-stranded RNA viruses of the Orthomyxoviridae family. There are three genera known to cause disease in humans: Influenza A, B, and C. The genome consists of eight segments for influenza types A and B, encoding at least 11 proteins, and seven for influenza C. Influenza A, B and C viruses produce an RNA polymerase complex, required for replication, along with surface glycoproteins critical for viral structure and host cell entry, and other viral polypeptides and accessory polypeptides.
Influenza A can be further categorized based on its hemagglutinin (HA) and neuraminidase (NA) proteins. The virus primarily infects epithelial cells in the upper respiratory tract, though it can spread to the lower tract and cause more severe illness.
Transmission occurs through respiratory droplets and direct contact with contaminated surfaces. Influenza is highly contagious and commonly associated with seasonal epidemics in temperate regions.
Symptoms typically appear rapidly and may include:
Although influenza often resolves without intervention in otherwise healthy individuals, it can cause severe, life-threatening complications in high-risk groups. These include older adults, children, pregnant individuals, and people with underlying health conditions. In severe cases, infection may lead to pneumonia and hospitalization.
As with other respiratory viruses, good hygiene practices—including handwashing, respiratory etiquette, and self-isolation—are effective in reducing transmission.
Vaccination remains the most effective prevention strategy. Seasonal vaccines are reformulated each year to match the most prevalent circulating strains. This is necessary due to ongoing antigenic drift—accumulation of mutations in the surface glycoproteins—which generates new variants. Because vaccine production takes at least six months, selecting the correct circulating subtype in advance is a challenge.
Influenza viruses have substantial zoonotic reservoirs, especially in birds and pigs. Due to their segmented genome, if two viruses infect the same cell, segments can reassort in a process called antigenic shift. This can give rise to novel variants capable of human infection.
Current influenza vaccines are strain-specific and may offer limited protection against future pandemic strains. Pandemic preparedness remains a global health priority due to the continued risk posed by emerging zoonotic influenza viruses.
