All influenza viruses have the potential to can change. It is possible that an avian influenza virus could change so that it could infect humans and could spread easily from person to person. Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If an avian virus were able to infect people and gain the ability to spread easily from person to person, an â€œinfluenza pandemicâ€ could begin.Â
An influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in several, simultaneous epidemics worldwide with enormous numbers of deaths and illness. With the increase in global transport and communications, as well as urbanization and overcrowded conditions, epidemics due the new influenza virus are likely to quickly take hold around the world.
Influenza A and influenza B are 2 of the 3 types of influenza viruses associated with annual outbreaks and epidemics of influenza. The third type, influenza C, causes only mild disease and has not been associated with widespread epidemics or pandemics. Annual outbreaks of influenza are due to minor changes in the surface proteins of the viruses that enable the viruses to evade the immunity humans have developed after previous infections with the viruses or in response to vaccinations.
Only influenza A virus can cause pandemics. When a major change in either 1 or both of their surface proteins occurs spontaneously, no one will have partial or full immunity against infection because it is a completely new virus. If this new virus also has the capacity to spread from person-to-person, then a pandemic is most likely to occur.
During the last century, 3 influenza pandemics caused millions of death worldwide, social disruption and profound economic losses. Influenza experts agree that another pandemic is likely to happen. Epidemiological models project that in industrialized countries alone, the next pandemic is likely to result in 57-132 million outpatient visits and 1.0-2.3 million hospitalizations, and 280 000-650 000 deaths over less than 2 years. The impact of the next pandemic is likely to be greatest in developing countries where health care resources are strained and the general population is weakened by poor health and nutrition.