Viruses need to infect living cells of other organisms in order to replicate their genetic material. When not in an infected living cell, they are considered independent particles, in the form of DNA surrounded by a protein coat. Only when a virus enters and infects a living cell, it can then start its reproductive cycle. It uses the host's (infected cell) replication system, to replicate its own DNA or RNA and create the necessary viral proteins to create more viruses. The newly created viruses are then able to infect other living cells.
