Viruses are incapable of reproducing without invading a host cell. What happens after that invasion depends on what is available in the host cell. The virus acts as though it is under the control of an on/off switch that responds to the availability of nutrients. If there are nutrients, the switch turns on, and the virus uses one form of reproduction called the lytic cycle. At this point, the virus is considered virulent, which means it is infectious and dangerous because it kills the host cells. 

If there are no nutrients available, the switch turns off and the virus uses another form of reproduction called the lysogenic cycle. At this point, the virus is considered temperate, which means it is not as dangerous because it is not killing the host cell--yet. Study the image below. Begin with step 1 and read each part to learn when and how the virus uses the lytic and lysogenic cycles to reproduce.