Answer: They must enter the cell of another organism.
Step-by-step explanation:
Viruses are intracellular parasitic organisms. It posses genetic material in the form of ribonucleic acid (RNA) which is enclosed inside a protein envelop. A virus being a parasite invade a host organism cell to produce it's own progeny viruses. A virus attach itself to the specific receptor site on the host cell membrane. The protein coat is left outside the cell and the viral genetic material is injected inside the host cell. Then replication of the viral genome takes place inside the host genome. The progeny viruses release from the host cell machinery by lysis of the host cell.