What do viruses actually do to the human body?
Viruses inject their DNA or RNA into body cells and re-program cells to make more viruses, and/or change into tumor cells.
Viruses are opportunistic proteins wrapped around nucleic acids, or genes.
They are mostly inactive until they enter human body cells. Then, voila--they become activated into major machines. These 'machines' will have inserted their nucleic acid--either the DNA or RNA--into the human cell's genome.
That means that new, 'foreign' DNA is now causing a change within the human cell's DNA. It is re-programming it. There are many results from this re-programming.
Some human cells become hibernator 'nests' for the virus. An example would be the herpes virus. Others will allow the virus to rapidly spread, or multiply. Some cells will gradually turn into tumor cells, for instance, with the human papilloma virus and genital warts.
Hope this helps ;)