An example of a biological agent that needs a host to replicate and spread is a smallpox. Smallpox is an airborne disease, can be transmitted from person to person through air by close contact, that is contagious caused by a variola virus. Before, it was one of the deadliest diseases known to mankind. The last devastating case was in Somalia in 1977 and was declared to be eliminated by World Health Organization in the year 1980.