Outside of a host cell, viruses do not use any energy. They only become active when they come into contact with a host cell. Once activated, they use the host cell's energy and tools to make more viruses. Because they do not use their own energy, some scientists do not consider them alive. Related Theory The only point favouring the virus as living entities is that they have the ability to reproduce (multiply) within the host by using host's machinery. The only point favouring the viruses as living entities is that they have the ability to reproduce (multiply)-though in the host using host's machinery.