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Intro to viruses

What a virus is. The structure of a virus and how it infects a cell.

Key points:

  • A virus is an infectious particle that reproduces by "commandeering" a host cell and using its machinery to make more viruses.
  • A virus is made up of a DNA or RNA genome inside a protein shell called a capsid. Some viruses have an external membrane envelope.
  • Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts.
  • Viruses reproduce by infecting their host cells and reprogramming them to become virus-making "factories."


Scientists estimate that there are roughly 1031 viruses at any given moment1. That’s a one with 31 zeroes after it! If you were somehow able to wrangle up all 1031 of these viruses and line them end-to-end, your virus column would extend nearly 200 light years into space. To put it another way, there are over ten million times more viruses on Earth than there are stars in the entire universe2.
Does that mean there are 1031 viruses just waiting to infect us? Actually, most of these viruses are found in oceans, where they attack bacteria and other microbes3. It may seem odd that bacteria can get a virus, but scientists think that every kind of living organism is probably host to at least one virus!

What is a virus?

A virus is a tiny, infectious particle that can reproduce only by infecting a host cell. Viruses "commandeer" the host cell and use its resources to make more viruses, basically reprogramming it to become a virus factory. Because they can't reproduce by themselves (without a host), viruses are not considered living. Nor do viruses have cells: they're very small, much smaller than the cells of living things, and are basically just packages of nucleic acid and protein.
Still, viruses have some important features in common with cell-based life. For instance, they have nucleic acid genomes based on the same genetic code that's used in your cells (and the cells of all living creatures). Also, like cell-based life, viruses have genetic variation and can evolve. So, even though they don't meet the definition of life, viruses seem to be in a "questionable" zone. (Maybe viruses are actually undead, like zombies or vampires!)

How are viruses different from bacteria?

Even though they can both make us sick, bacteria and viruses are very different at the biological level. Bacteria are small and single-celled, but they are living organisms that do not depend on a host cell to reproduce. Because of these differences, bacterial and viral infections are treated very differently. For instance, antibiotics are only helpful against bacteria, not viruses.
Bacteria are also much bigger than viruses. The diameter of a typical virus is about 20 - 300 nanometers (1 nm = 10-9 m)4. This is considerably smaller than a typical E. coli bacterium, which has a diameter of roughly 1000 nm! Tens of millions of viruses could fit on the head of a pin.

The structure of a virus

There are a lot of different viruses in the world. So, viruses vary a ton in their sizes, shapes, and life cycles. If you're curious just how much, I recommend playing around with the ViralZone website. Click on a few virus names at random, and see what bizarre shapes and features you find!
Viruses do, however, have a few key features in common. These include:
  • A protective protein shell, or capsid
  • A nucleic acid genome made of DNA or RNA, tucked inside of the capsid
  • A layer of membrane called the envelope (some but not all viruses)
Let's take a closer look at these features.
Image modified from "Scheme of a CMV virus." by Emmanuel Boutet, CC BY-SA 2.5. The modified image is licensed under a CC BY-SA 2.5 license.

Virus capsids

The capsid, or protein shell, of a virus is made up of many protein molecules (not just one big, hollow one). The proteins join to make units called capsomers, which together make up the capsid. Capsid proteins are always encoded by the virus genome, meaning that it’s the virus (not the host cell) that provides instructions for making them.
Capsids come in many forms, but they often take one of the following shapes (or a variation of these shapes):
  1. Icosahedral – Icosahedral capsids have twenty faces, and are named after the twenty-sided shape called an icosahedron.
  2. Filamentous – Filamentous capsids are named after their linear, thin, thread-like appearance. They may also be called rod-shaped or helical.
  3. Head-tail –These capsids are kind of a hybrid between the filamentous and icosahedral shapes. They basically consist of an icosahedral head attached to a filamentous tail.
    Image modified from "Non-enveloped icosahedral virus," "Non-enveloped helical virus," and "Head-tail phage," by Anderson Brito, CC BY-SA 3.0. The modified image is licensed under a CC BY-SA 3.0 license.

Virus envelopes

In addition to the capsid, some viruses also have an external lipid membrane known as an envelope, which surrounds the entire capsid.
Viruses with envelopes do not provide instructions for the envelope lipids. Instead, they "borrow" a patch from the host membranes on their way out of the cell. Envelopes do, however, contain proteins that are specified by the virus, which often help viral particles bind to host cells.
Image modified from "Enveloped icosahedral virus," by Anderson Brito, CC BY-SA 3.0. The modified image is licensed under a CC BY-SA 3.0 license.
Although envelopes are common, especially among animal viruses, they are not found in every virus (i.e., are not a universal virus feature).

Virus genomes

All viruses have genetic material (a genome) made of nucleic acid. You, like all other cell-based life, use DNA as your genetic material. Viruses, on the other hand, may use either RNA or DNA, both of which are types of nucleic acid.
We often think of DNA as double-stranded and RNA as single-stranded, since that's typically the case in our own cells. However, viruses can have all possible combos of strandedness and nucleic acid type (double-stranded DNA, double-stranded RNA, single-stranded DNA, or single-stranded RNA). Viral genomes also come in various shapes, sizes, and varieties, though they are generally much smaller than the genomes of cellular organisms.
Notably, DNA and RNA viruses always use the same genetic code as living cells. If they didn't, they would have no way to reprogram their host cells!

What is a viral infection?

In everyday life, we tend to think of a viral infection as the nasty collection of symptoms we get when catch a virus, such as the flu or the chicken pox. But what's actually happening in your body when you have a virus?
At the microscopic scale, a viral infection means that many viruses are using your cells to make more copies of themselves. The viral lifecycle is the set of steps in which a virus recognizes and enters a host cell, "reprograms" the host by providing instructions in the form of viral DNA or RNA, and uses the host's resources to make more virus particles (the output of the viral "program").
For a typical virus, the lifecycle can be divided into five broad steps (though the details of these steps will be different for each virus):
  1. Attachment. The virus recognizes and binds to a host cell via a receptor molecule on the cell surface.
  2. Entry. The virus or its genetic material enters the cell.
  3. Genome replication and gene expression. The viral genome is copied and its genes are expressed to make viral proteins.
  4. Assembly. New viral particles are assembled from the genome copies and viral proteins.
  5. Release. Completed viral particles exit the cell and can infect other cells.
The diagram above shows how these steps might occur for a virus with a single-stranded RNA genome. You can see real examples of viral lifecycles in the articles on bacteriophages (bacteria-infecting viruses) and animal viruses.

Want to join the conversation?

  • blobby green style avatar for user Dave Mac
    Will science/medicine ever find a way to make us immune to every virus and bacteria?
    (22 votes)
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    • leaf green style avatar for user juyeonyang0717
      Another aspect is that we are constantly running into new viruses. Why? Viruses can be divided into two groups: one group of viruses spreads only between humans, and the other group mainly spreads between animals and may spread to humans. Drastic population increase and worldwide drought led to widespread deforestation, in order to acquire more farming land and wood. Naturally, many animals lost their habitats and were forced to move into lands where humans lived. Viruses that were originally spread only between these animals were able to spread to humans as well. So if the status quo continues, humanity is going to come across much more diverse viruses than ever before. It can be assumed that it won't be easy to find a cure for all these new viruses.
      (17 votes)
  • starky seed style avatar for user aj
    Can viruses be considered a separate kingdom from the main five kingdoms of living organisms?
    (27 votes)
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  • starky sapling style avatar for user Vivin Kannan
    How did Viruses develop in the first place? Did they just evolve or something......
    (21 votes)
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  • blobby green style avatar for user Don Rowan
    If a surface, e.g., a table, is contaminated with a virus, e.g., COVID 19, is the virus unit attached to some carrier unit such as inside a water vapor droplet or attached to dust, etc. or is it a separate chemical unit free to be moved about like a stone laying on the ground that can be kicked around?
    (14 votes)
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    • leaf grey style avatar for user masterYoda
      I think "chemical unit" is the wrong phrase to use here. A simple stone or pebble at a microscopic scale can be compared to a rough mountain range on earth. At that scale, it's basically the same as any other similar surface. A small pebble or a giant boulder is the same at that level.

      On all surfaces, there are a lot of places for germs to hang out, but it depends on the germ and the surface whether you can actually find anything there.

      Usually, the answer is both.
      Viruses can most definitely stick around, even on smooth, dry surfaces for hours or even days, and water vapor and dust are great examples of where to find pathogens.

      But again, it depends on the surface and the virus.
      (4 votes)
  • old spice man green style avatar for user Jahnavi  Desai
    How is a viral infection treated?
    (4 votes)
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    • female robot grace style avatar for user YouAreEnough
      Viral infections are hard to treat because viruses live inside your body's cells. They are "protected" from medicines, which usually move through your bloodstream. Antibiotics do not work for viral infections. There are a few antiviral medicines available so far.
      Hope this helps!
      (10 votes)
  • duskpin tree style avatar for user nataliajan101
    Could there be a way to attract viruses to certain areas? Because it was noted in the article that a virus binds to receptors such as proteins.
    Is it possible to get the virus out of the host cell without causing harm to the host cell?
    Can some host cells be immune to viruses or unaffected by even the worst of viruses?
    (7 votes)
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    • leaf grey style avatar for user masterYoda
      It would be possible, but not practical. Having free roaming proteins in your body to attract viruses is like having pools of acid in the ocean to digest plastic.
      It works, but not particularly well and causes more harm than good.

      Also it's worth noting once a virus is inside a cell, its not coming out until thousands of copies of itself are ready to burst out.

      And as far as we know, a virus-immune cell isn't quite biologically possible.

      Hope this helps
      (4 votes)
  • leaf blue style avatar for user cellogirl04
    I know you told us about the life cycle of a virus in the above lesson. But I was wondering about the life cycle of a virus with growth (development) and death.
    Do viruses die?
    Do they grow/develop - along with reproducing?
    (5 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      No, viruses do not have that trait of living organisms - growth and development.

      Viruses only reproduce - if they find host (cell).

      Do viruses die? No. They just don't reproduce in case they do not find a suitable host (meaning wasted proteins and nucleic acids.

      Moreover, prions (just proteins) may cause a ruckus if in human body (thought to lead to Alzheimer disease).

      Think of viruses rather as machinery not living organism.
      (6 votes)
  • blobby green style avatar for user kailacqua
    Why are viruses smaller than their host?
    (5 votes)
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  • blobby green style avatar for user 970078416
    can a virus be killed by freezing it
    (5 votes)
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    • duskpin ultimate style avatar for user Liam
      No the scientific community is split on whether a virus is alive or not but freezing a sample will actually preserve it. Researchers have found and analyzed ancient samples of viruses taken from glacial deposits. Hope this was helpful. :)
      (5 votes)
  • blobby green style avatar for user william
    If a virus meets a hydrogen peroxide molecule, what happens?
    (4 votes)
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