A virus is a parasite that needs living host for its survival. Such parasites can live within the host’s body and feed on its nutrition. Many viruses often cause harm to the host by weakening host’s immune system and then subject them to the infection. If such infections are contagious, then it spreads to the other healthy hosts. Virus enters the other hosts through various portals of entry and sets the same infection that weakens the host’s immune system. Many times, such contagious infections prove to be fatal and cause death of the host. Since such contagious virus limit the size of the population and since they easily spread in the dense population, it is often considered as the density-dependent factor.