What are viruses?
DNA or RNA contained within a protein shell.
Viruses are set up so that they can inject DNA or RNA into a living cell to hijack the cells production chain.
The protein shell around the DNA is simply a mechanism to enter the host cell. Once the DNA or RNA is in the host cell the protein floats away (Not only do viruses make us sick, they litter in us too!)
The DNA or RNA then changes what the cell is producing from whatever it's true purpose was, into a virus maker. The cell continues to make copies of the virus until it explodes and spreads those new viruses out into its surroundings.