Flu virus particle. Computer artwork of an influenza (flu) A virus particle (virion). The virus consists of a core of RNA (ribonucleic acid) genetic material surrounded by a protein coat (black). Embedded in the coat are the surface proteins haemagglutinin and neuraminidase. Both surface proteins are associated with the pathogenicity of a virus. Haemagglutinin binds to host cells, allowing the virus to enter them and replicate. Neuraminidase allows the new particles to exit the host after replication.