About.com explains that viruses multiply by overtaking a host cell and using the cell’s internal machinery to produce many new viruses, which are then released into the environment. These new viruses may then infect other cells, causing these newly infected cells to produce even more viruses.
Viruses are obligate, intracellular parasites that cannot carry out their lifecycle without the benefit of a host cell. About.com explains that virions, or single virus particles, are benign objects that lie dormant until they gain entry into a cell. This is because viruses are very simple organisms, which lack most of the components necessary to multiply. Florida State University explains that viruses are primarily composed of a small amount of genetic material and a protein coat that covers this DNA or RNA.
Most cells use organelles called ribosomes to construct proteins based on the genetic code of the cell. Viruses do not have ribosomes, so they use the ribosomes of the host cell to construct the proteins they need, according to About.com. Some viruses carry their genetic code in a single-stranded RNA molecule, while others utilize a double-stranded DNA molecule. Those that use DNA must often gain access to the host cell’s nucleus before replicating. By contrast, RNA-based viruses engage in replication within the cytoplasm of the host cell.