Viruses are the smallest and simplest type of infectious agents. They are not composed of cells, but consist only of genetic material wrapped in a protein coat.
Viruses cause infection by entering a cell, releasing their genetic material into it, and then using the host cell's DNA and metabolism to create many, many copies of this protein-coated material.
These activities usually cause the rapid death of the host cell, and it disintegrates, releasing all the duplicate viruses to infect more and more cells nearby.
Bodily defence against viral infection is via its immune response.
Viruses which are able to evade this response, such as HIV, do so by producing repeated changes to their protein coat by evolution.