A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus at an extraordinary rate. Viruses can infect all types of life forms, from animals and plants and even microorganisms. With such simple structures, you would think that viruses would be quite vulnerable and easy to kill. However, many of the popular products we use to kill bacteria fail to destroy viruses. Why are they so hardy?