How Does a Virus Work? Viral Mutations, Antigenic Shift and Drift


Home / How Does a Virus Work? Viral Mutations, Antigenic Shift and Drift

The influenza virus has eight genes that are part of its makeup. Image by: NIAID

Antigenic Drift

So how does a human become infected with the bird flu if it is normally found in birds? This is the result of antigenic drift.

Remember, antigenic drift occurs as the virus changes its makeup over time so that it can continue to reinfect people. Well, as it continues to change, it can also cross the species barrier to humans. The influenza virus has had a good track record for crossing from birds to humans (avian flu) and from pigs to humans (swine flu).

The influenza virus has eight different components to its makeup, which allows for many different combinations of viruses that include swine, avian, and human flu. Pigs are particularly good at harboring these viruses because they have receptors for avian, swine, and human flu. This means that if pigs are around birds or people that have the flu, then the flu virus from the human or the bird can enter into the pig and get mixed up. This can lead to a new (novel) hybrid influenza virus.

When animals and humans live in close quarters, the passing and mixing of viruses can happen quite easily, resulting in humans becoming sick with the swine flu or the bird flu. This is also known as adaptive mutation.

Antigenic Shift

This is how the antigenic shift works. Image by the NIH.

There is one more way that viruses can change and cause a pandemic, through a process called the antigenic shift. This can occur in one of two ways.

  • One way this can occur is from a flu strain crosses the species barrier and makes a new virus; like what we talked about in the above section.
  • The other way occurs when the production of the virus changes its proteins and results in a new virus. This is an abrupt and major change and most people have no immunity to this new virus, which can result in pandemics.

However, unlike antigenic drift, antigenic shift only happens occasionally.

Viruses, Antigenic Drift, and Antigenic Shift

So there is your brief lesson on how viruses work, how they change, and how they can cause pandemics. This background information is helpful so that you can better understand what happens when pandemics occur- or when new strains emerge like the H7N9 bird flu in China or the coronavirus that has a extremely high mortality rate. Keep yourself and your family healthy with knowledge.


Centers for Disease Control and Prevention. How the flu virus can change: antigenic shift and shift. Accessed April 19, 2013.

Neiman Foundation. Crossing the Species Barrier. Accessed April 18, 2013.

World Health Organization. How pandemic influenza emerges. Accessed April 19, 2013.

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