Viruses are unique among infectious agents because they are?

Answer 1

First of all, viruses are not technically living (pathogenic bacteria is). They infect by injecting a part of their DNA into the host cell which responds by producing more viruses until the host cell lyses (essentially explodes) and the viruses go on to infect other cells. The DNA injected can also remain dormant until an environmental trigger stimulates expression of the specific gene that was injected by the virus. Also viruses have no way of checking for errors when they replicate their DNA so they can easily have various mutations that allow them to respond to dangers in their environment much more easily than bacteria. For this reason, in many cases once an organism has been infected with a viral DNA it cannot be cured (examples are HIV and rabies). Vaccines help to prevent the initial infection of the host cell but can do nothing once the virus has taken hold.