What is the flu? Influenza or “flu” for short is a virus. Viruses contain either RNA or DNA but never both in one viral cell. They work by entering human cells via adsorption and exploiting the functions of the nucleus, in the healthy human cell, to replicate the nucleic acid and place it inside capsules called nucleocapsids which are then budded off and released out of the cell to go onto infect more healthy cells in the body; a vicious cycle.