A virus particle is not a complete cell but an intracellular parasite. Hence, it cannot reproduce without the help of a host cell. Once inside a host cell, the virus is made in such a way that it replicates itself. There are two ways in which viruses reproduce or multiply their numbers: the lytic cycle and the lysogenic cycle.
In the lytic cycle, a virus particle attaches itself to a host cell and infuses its nucleic acid into the host cell, effectively taking the cell hostage. Inside the cell, the virus begins replicating itself, and the cell is soon filled with the virus and bursts like a balloon with too much air. The new virus particles then attach themselves to nearby cells and start the lytic cycle once again.
In the lysogenic cycle, however, the viruses do not replicate as quickly. These viruses enter the host cell, and the nucleic acid of the virus becomes part of the chromosome of the cell. Although the nucleic acid of the virus is present in the chromosome of the cell, the virus is not active at that point, and it does not affect the functions of the cell. When the host cell replicates, the viral nucleic acid also replicates itself. Eventually, the nucleic acid leaves the chromosome and takes over the cell and in the process kills the cell.