What are Viruses?

Image credit:CDC/ Alissa Eckert, MS; Dan Higgins, MAMS

We all heard of the term “virus” but what exactly is a virus?

A virus or virion is an infectious agent. Unlike other infectious agents such as bacteria, viruses are not cellular in nature and lack essential sophisticated mechanisms for their own reproduction.

For a virus to multiply, it must first get into a living host cell. The host can be of plant, animal or human origin. There are even viruses that infect bacteria. Once inside a host cell, viruses can replicate by using the cell’s own building materials (amino acids and nucleotides), energy source (in the form of ATP) and in-house synthesis machinery to produce viral proteins and nucleic acids (DNA or RNA) based on the genomic blueprints from the virus particle. The end products can then be used to assemble new viruses. Basically, viruses use the host cells to photocopy themselves.

Are viruses alive?

This is rather a philosophical question than a scientific question. After years of scientific research, we now know the molecular details of virus. Virus itself has no metabolic activity and cannot replicate outside host cells. They can be considered nonliving entities. However, viruses contain genomic blueprints (DNA/RNA sequences) that can be used for their replications making them infectious. Like other organisms, viruses also evolve (mutate) and sometimes very rapidly for them to adapt to the new host and new environments. Some might consider viruses alive because of these reasons. However, not all viruses are infectious. Inactivated viruses (used in some vaccines) are not capable of initiating a complete replication-cycle. Hence, it is not infectious. Due to the diverse nature of viruses, they can be considered living and non-living. What do you think?

Now let’s take a look at the size of viruses. Viruses are tiny and can’t be seen by the naked eye. Most viruses can only be observed with an electron microscope (EM). In general, an EM can produce magnifications of at least 100 times greater than a typical light microscope used for visualising cells.  

Credit: Science/ Mitch Leslie

As you can see from the 3D illustration above, viruses vary in size. The biggest virus currently known is Pithovirus, 3x bigger than the Mimivirus, shown in the image. Pithovirus is almost as big as a bacterium and bacteria are much smaller than a human cell. Both Pithovirus and Mimivirus infect amoebas. What about the smallest virus currently known? The smallest virus is the porcine circovirus. It is approximately 17nm in diameter. Porcine circovirus is known to infect pigs.

Take a look at bacteriophage in the images above. As indicated by its name, bacteriophage infects bacteria. Look at the structure of a bacteriophage, doesn’t it look like a tiny robot straight out of a sci-fi film. Now, look at the other image, you can see multiple bacteriophages attaching themselves to the bacterium. You can even see some of them inside the bacterium.

Compare the sizes of viruses, cells, coffee bean and a grain of rice using this interactive tool by Genetic Science Learning Center of the University of Utah.

Inside these tiny virus particles, there are DNA/RNA protected by a layer of proteins. It’s hard to imagine how such tiny particles can claim millions of human lives throughout history. One of the deadliest epidemics in human history, the Spanish flu was caused by the influenza virus. For now, let’s take a look at some images of some of the most well-known human viruses.

Stay tuned for future posts on virus structure and assembly.

About the Author

Ameline Lim, Ph.D.
Ameline Lim, Ph.D., is a research scientist and biologist. She received her PhD from the University of New South Wales at Sydney, Australia. She has broad interests in medicine, history of science, and behavioural science. She is particularly interested in science education and the public understanding of scientific research. Her blog 'Science with Amy' celebrates the pleasures of finding things out and general curiosity.