Viruses cannot grow or reproduce, but they can manufacture copies of themselves by stealing protein mass and other nutrients from host cells. Viruses propagate exponentially, given adequate material with which to work.
Viruses create more of themselves by repurposing the cells they infest. The cells are forcibly reconfigured into producers of more of the infesting virus, their original functions either compromised or destroyed entirely. This process is what makes some viruses so potentially deadly to humans and other animals.
Viruses incorporate either DNA or RNA in their nucleic acid. Some are so simple and small that they encode only four proteins, while others are staggeringly complex, incorporating 100 to 200 encoded proteins. The huge breadth of diversity in virus structure and complexity makes it difficult to combat their predation of human cells and their extreme facility for destroying cellular material makes them the source of many common afflictions, some deadly or horrific, others commonplace.
Viruses are widely used in laboratory research. Cells taken over by viruses provide huge insights into the workings of cellular material and the ways in which cells can be modified, healed and broken down. They are used in gene therapy techniques to deliver payloads of genetic material to cells and to reconfigure those cells to produce more of said material.