Influenza is a very concerning virus for public health scientists. Because it can be transmitted easily between people, cause severe disease, and leave the host prone to other respiratory complications, influenza could cause a deadly worldwide pandemic. One of the worst pandemics in recorded history was caused by influenza virus in 1918. The Spanish Flu caused an estimated 20 to 40 million deaths worldwide. It caused severe disease in young adults, which is not common with most flu viruses. The most susceptible populations are normally the very young, and the very old. In addition, the virus left many susceptible to secondary infections, such as pneumonia. The virus was so severe, the average life expectancy in the US dropped by 10 years. Part of the reason why the pandemic may have spread so quickly was due to international travel. Many soldiers, who had been living in close quarters during World War I, suddenly returned to their homes all over the world, bringing the virus with them.
Influenza is particularly prone to causing such devastating pandemics due to the nature of its genome. The genetic information is stored in 8 separate RNA molecules. Because RNA polymerase has a lower fidelity that DNA polymerase, the genome of influenza is very susceptible to mutation. This is called genetic drift. When enough mutations accumulate, it can make the proteins produced by the virus difficult for the host immune system to recognize, and prevent the virus from being attacked. In addition, some influenza viruses can infect multiple species. If a virus that can infect both humans and birds is in a bird, it can exchange some of the RNA genome with another, avian-origin influenza virus. This results in drastically new viruses, which can be unrecognizable by the human immune system. This process is called genetic shift. Genetic drift and genetic shift are some reasons why public health officials were particularly worried about the 2009 H1N1 influenza virus. It had origins from both human and swine influenza viruses, which indicated that the virus may not be easily recognized by the human immune system. In addition, many of those who had severe disease from the virus were young adults, which is similar to the 1918 Spanish Influenza outbreak.
Not every influenza virus is able to cause a pandemic. Most viruses that have circulated through the human population recently are not candidates for a severe pandemic, as many people would have adaptive immune responses to the virus. Rather, viruses from a different animal source, such as pigs or birds, are much more likely to cause a pandemic. These would not be rapidly recognized by the immune system, and would be able to cause more damage to many people before being controlled by adaptive immune responses.
The recent outbreak of avian influenza in China is causing concern to public health officials. The H7N9 avian influenza virus has so far infected nearly 90 people, and has been fatal in 17 of these cases. What is concerning to investigators is how the virus is being transmitted. Estimates state that as many as forty percent of the patients have not had direct contact with poultry, which indicates that the virus might be able to spread from human to human. However, due to the processing of poultry in markets in China, it is possible that these patients indeed contracted the virus directly from birds. Chinese poultry markets are often crowded, with many droppings accumulating, and the processing of fowl in preparation for the consumer can result in droplets being dispersed. Even if a patient had not directly been in contact with the poultry, they could have inhaled these droplets in the market and become infected.
The H7N9 virus is concerning to public health scientists because of its avian origins. The virus would be novel in the human population, meaning that no one would have memory immune responses against the virus. Without any prior protection in the population, the virus would be free to spread to virtually any human on earth. With rapid travel available between almost every location on earth, a pandemic could quickly spread throughout the world, causing rapid devastation. Public health officials are constantly watching and monitoring any novel influenza virus that is reported, in hopes of preventing a global pandemic.