Once a virus enters your body, what will it do?
To put it simply, viruses hijack your somatic cells, incorporate their viral genetic material into it and use the biological mechanisms of the cell to reproduce.
Viruses are obligate parasites meaning they cannot survive (does not show any semblance of life) without a living host. Most viruses are just strands of genetic material (DNA or RNA) that is enveloped by a protein coat.
To proliferate themselves, viruses seek a living host, incorporate themselves to the host's cell, insert their genetic material to the genome of the cell thereby making the cell work for the viral particle. This particular process gives rise to abnormalities in the cell function and in a larger perspective, translates to a viral disease.