Answer
Viruses can't do much by themselves, without a host they are just genomes in a protein coat. Once they acquire a host, however, they "come alive" because the organism gives the virus everything it needs to work. The virus then uses the host cell's internal mechanisms to reproduce.
Work Step by Step
This statement is saying that viruses can be useful or useless, in the metaphorical twilight zone of life because they can be either. It all depends on whether or not they have a host cell. If they are outside a cell they aren't doing much, they're effectively useless because they're just "dead assemblies of molecules." But when they are inside a cell "they are alive," replicating and using their host cell to infect the surrounding cells.
