Influenza virus (Flu) is a type of enveloped, single-stranded negative-sense RNA virus belonging to the family Orthomyxoviridae. Based on antigenic differences in its nucleoprotein and matrix protein, the virus is primarily classified into four types: A, B, C, and D. Among these, influenza A and B viruses are the main pathogens responsible for seasonal influenza in humans. The influenza virus exhibits significant genetic diversity and antigenic variability. Its genome consists of multiple segments, which are prone to reassortment and point mutations, leading to the continuous emergence of new viral strains. This enables the influenza virus to spread persistently worldwide, posing periodic challenges to public health and underscoring the importance of ongoing global surveillance and virological research.

Influenza virus particles typically exhibit a spherical or pleomorphic morphology, with a diameter of approximately 80–120 nm. The viral genome consists of segmented single-stranded negative-sense RNA. Influenza A and B viruses contain eight RNA segments, encoding at least ten proteins. The entire viral particle is structured from the inside out, primarily comprising the ribonucleoprotein complex (RNP), the matrix layer, and the viral envelope. Each RNA segment binds to the nucleoprotein (NP) and three RNA polymerase subunits (PA, PB1, PB2), forming an independent RNP core responsible for viral transcription and replication. The matrix layer is mainly composed of matrix protein 1 (M1), which plays a critical role in maintaining viral structural stability, mediating RNP nuclear export, and facilitating viral budding. The viral envelope is derived from the host cell membrane and is embedded with two important surface glycoprotein spikes: hemagglutinin (HA) and neuraminidase (NA). HA is responsible for recognizing and mediating the binding of the virus to sialic acid receptors on the host cell surface, as well as facilitating membrane fusion. It is a key determinant of the virus's host range and tissue tropism and serves as the primary target for inducing neutralizing antibodies. NA, on the other hand, cleaves sialic acids to promote the release of progeny viruses from infected cells. The antigenic variability of these two proteins serves as the molecular basis for the virus's ability to evade host immunity and cause seasonal epidemics and pandemics.

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