Pandemic-causing viruses have three characteristics in common. The virus:

• Has the ability to reproduce rapidly and spread through human-to-human contact. Infected members of the population unknowingly incubate the disease and spread it to others.
• Has the ability to mutate rapidly. The potential for a virus to undergo a natural mutation after release into the population is higher for pandemic viruses than for other influenzas.
• Causes a “second wave” of victims. The effect of a highly virulent strain of virus causes a second series of victims, often among healthcare workers and others who are working to contain it.