Flu viruses, Michigan strain, illustration - stock illustration

Flu viruses, conceptual illustration demonstrating seasonal character of respiratory infections with increase of flu incidences in cold rainy weather. Each virus consists of a core of RNA (ribonucleic acid) genetic material surrounded by a protein coat (blue). Embedded in the coat are surface proteins (spikes). There are two types of surface protein, hemagglutinin (purple) and neuraminidase (orange), and each exists in several subtypes. Both surface proteins are associated with the pathogenicity of a virus. Hemagglutinin binds to host cells, allowing the virus to enter them and replicate. Neuraminidase allows the new particles to exit the host after replication.