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New influenza viruses capable of causing a pandemic emerge every few decades and attack the human population causing illness and death. Eventually after a virus has been around for a while, the human population builds up an immunity to it, and the influenza can not longer spread easily within a population. So where does the next pandemic strain come from? To understand this, you have to spend some time learning about the influenza life cycle.
The influenza virus genome contains 8 pieces of single-stranded RNA (ssRNAs) that code for the 8 proteins of the virus. These proteins carry out the various functions of the virus, copying its DNA, binding to and entering host cells, etc. The two proteins that the host immune system recognizes and attacks are hemagluttanin (HA) and neuraminidase (NA). RNA viruses such as influenza make many mistakes when copying their genomes. Most of these mistakes are detrimental and a virus that gets that errant copy, cannot replicate, but some of the mistakes cause them to not be as easily recognized by the host immune system, yet still carry out their function for the virus. When enough of these little changes build up in a gene, the virus can successfully infect even hosts who have been exposed to an earlier type of the virus. In many cases this type of change will cause mild illness because the host has seen something very similar to this virus before and often can fight it effectively.
However, influenza has one more nasty trick up its sleeve. Remember that the genome is composed of 8 ssRNAs (indicated as HA, NA, M, P, NS, NP, PB1, and PB2). If two viruses infect a host cell at the same time, during packaging these ssRNAs will be mixed and matched. So a resulting strain could have HA, NA, PB1 and NP from one strain and M, P, NS and PB2 from the other or any combination of the two. All this mixing and matching may result in a non-functional virus, but it may also result in a new virus that the host immune system cannot recognize at all. This appears to be what has happened with the new H1N1 virus that has been discovered. Sequencing of its genome has shown that 6 of the genes (HA, P, NS, NP, PB1, and PB2) of the virus are from a H1N1 strain that has circulated in the U.S. for years, but two of the genes (NA and M) are from a Eurasian swine influenza virus.
Most alarming about this H1N1 strain is that it appears to be capable of transmission from person-to-person, and recent reports are showing that the virus is moving around the globe as we speak. The mortality rate among patients with influenza so far appears to be low, but its too early to tell what the real rate will be. The good news is that the virus is susceptible to oseltamivir and zanamivir, two neuraminidase inhibitors. Availability of these drugs should decrease the death rate from H1N1. It will also be interesting to see how effective communication and coordination among governments will be in slowing the viruses spread. If a true pandemic can be delayed until a enough vaccine is available for wide-spread vaccination, this pandemic could be stopped before it starts. If you want to learn more about the influenza life cycle, check out the section on it in the textbook.