Evolution of the Flu Virus

Viruses must evolve at rates faster than the rates at which the hosts evolve; otherwise these viruses would no longer exist. As a host cells evolve so that a virus can no longer infect them by changing their surface receptors so that the virus can not attach, the virus adapts for this change their surface proteins so that they can attach to the changes host cells. Also, viruses can not survive if their hosts become extinct. Thus, it is beneficial for a virus to be able to cross over to a new host as this will give them a greater chance for survival. This means that if one of their potential hosts becomes extinct, then the virus can continue to infect the other host they have evolved to infect. These are some of the reasons that evolution of flu viruses is so vital to their continuation.

For example, the influenza virus mutates so that every year it can continue to infect its hosts. The influenza virus has an “H” and “N” protein. When humans are infected, we make memory antibodies to fight off the influenza virus, if and only if it is the same flu we were previously infected with. As the flu spreads throughout the world, it mutates and changes. The RNA that codes the “H” and “N” proteins, if mutated will be slightly different; if one of them does mutate, then our antibodies can no longer bind to the virus. It is unlikely that both the “H” and the “N” proteins will mutate in a given year; generally, one of them does mutate. When we are exposed to a flu virus with only one mutated protein, some of our antibodies will bind and the others will not. When this happens the infection will be mild, but it will generally affect a large number of people in the population. This process of ongoing mutation is referred to as antigenic drift.

In addition to mutating the “H” and “N” proteins, the flu virus can also rearrange its RNA by combining with other flu viruses. These hybrid flu viruses have a new set of “H” and “N” proteins and no people have antibodies to protect themselves from these viruses yet. These types of hybrid viruses have the potential to infect a large amount of people very fast. It is also possible for flu viruses to spread from species to species. If a human picks up a flu virus from an infected animal, such as a pig, horse, chicken, or duck, they may experience mild symptoms, but this virus will no spread to other humans. If however, this human is also infected with the human flu virus, these two viruses can cross and produce a virus that has rearranged RNA molecules; there is a large chance for antigenic shift with this new evolved version of the flu virus. The likelihood of this occurring is quite slim, but it does happen approximately every 16 years.

The Center of Disease Control (CDC) monitors the world for antigenic drift and shift so that they can predict how bad a flu season will be. If antigenic drift is detected, then we are in for a bad flu season and it is recommended that those with weak immune system get vaccinated. If antigenic shift is detected, then precautions are taken so that a pandemic does not occur. This is how the human population tries to fight off the evolution and adaption of the flu virus, since it is able to change must faster than we are.

This website shows flu activity over a given period of time.
Here are some images of the flu.