Unlike bacteria, a virus is not a complete organism and cannot survive on its own. “It needs a host and then it acts like a serial killer,” Dr. Pawan Grover says. It can use the host’s DNA make, which increases the risk of mutation. This is what makes viruses difficult to contain, he adds. Some are more lethal than others if the factors are right, Dr. Brent W. Laartz, a board certified infectious diseases specialist, says. Viruses must invade cells of a living host to reproduce and if a person has no immunity, the result can be deadly.