Flu viruses, computer illustration. Each virus consists of a core of RNA (ribonucleic acid) genetic material surrounded by a protein coat. Embedded in the coat are surface proteins (spikes). There are two types of surface protein, haemagglutinin and neuraminidase, and each exists in several subtypes. 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.