My son has the flu.  He is feverish and miserable.  His misery stems from the fact that the cells of his body have been commandeered by the influenza virus and have been forced to turn into mini virus factories.  It blows my mind that a virus so small that it can’t be seen without an electron microscope can easily overpower my 6 foot tall son and put him out of commission for days. Viral infections are notoriously hard to treat, given the peculiar “life” cycle of a virus.

Are Viruses Alive?

Scientists use several criteria to classify things as living or non-living.  Depending on what source you use,  there are between five and eight different characteristics that living things all share.  Whether all eight characteristics must be met is debatable, but nearly every scientist agrees that in order to be considered alive, an organism must:

  1. Be composed of one or more cells
  2. Contain DNA
  3. Obtain and use energy from the environment
  4. Sense and respond to environmental stimuli
  5. Reproduce

Viruses don’t meet most of these criteria.

Every living thing is made up of cells.  Numerous organisms (including humans) contain many cells while other organisms exist which are composed of only one cell (including bacteria and protists).  Viruses, on the other hand, are acellular: they are not made up of cells. A virus is little more than genetic material encased in a protein coat.  (Some viruses are further wrapped inside a membrane, absconded during their release from an infected host cell.) While some (but not all) viruses contain DNA, others carry their genetic information as RNA. Viruses do not obtain or use energy or sense and respond to environmental stimuli.  While viruses do reproduce to form new viruses, they can only do so by infecting host cells. Therefore, scientifically-speaking, viruses are not living things. How then can they cause disease?

Viral Infection

Amazingly, every living thing on the planet is susceptible to infection by at least one type of virus. Viruses exist that are able to infect humans (influenza, herpes), plants (tobacco mosaic virus, leaf curl virus), animals (rabies, cowpox virus), insects (entomopoxvirus, densovirus), fungi (mycovirus), amoeba (pandoravirus), algae (Phycodnaviridae), and bacteria (bacteriophage). Despite the multitude of different viruses on the planet, every virus shares one thing in common:  they are unable to replicate unless inside of a host cell. Because viruses are acellular, they lack the cellular components necessary for life. Instead, during a viral infection, a virus “hijacks” the host cell machinery and turns the cell into a factory with one job: to create more virus. There are a variety of ways that viruses employ to replicate, but every virus uses the host cell organelles, energy, and resources to make new virus particles. The progeny virus released from the host cell can then infect a new cell, beginning the cycle anew.  

New virus particles released from a lysed host cell following a lytic infection
Viruses released from a host cell during lysis

What is the fate of an infected cell? It depends on the virus. Some viruses—including members of the rhinovirus family (a cause of the common cold)—trigger a lytic infection. Following a lytic infection, the host cell dies by lysis—literally breaking open to release newly-formed virus.

New virus particles budding from an infected host cell
New virus particles budding from an infected cell

Other viruses “bud” from the cell, taking components of the host cell membrane with them.   This process does not kill the infected cell immediately, thus the cell continues to produce more progeny virus.  Infected cells do eventually die as a result of expending too much energy and resources propagating virus and too little on maintaining normal cell functions. This is the process used by the influenza virus, the virus that causes the flu.  

Still other viruses, including those belonging to the herpes family, employ yet another method. Following entry into a host cell and an acute round of infection, the viral genome integrates within the infected cell DNA. In this state, termed a latent (or lysogenic) infection, no acute disease symptoms remain.  This lysogenic infection can persist for years. However, each time the host cell replicates its DNA, it copies the integrated viral DNA too. When the cell divides, all cells produced from the original cell will also contain the integrated viral DNA. Following an environmental trigger (illness, stress, etc.), the virus can emerge from its latent state and force another round of active infection. This is why cold sores (caused by a herpes virus) reemerge in the same spot, year after year. The absence of a cold sore signals a latent infection when the virus is silently hidden in the host cellular DNA. During a cold sore eruption, the virus has re-activated and is causing a lytic infection.

This video does a great job summarizing viral infections.

How to Treat a Viral Infection

While prevention is the best medicine, it is nearly inevitable that you will at some point become infected with a virus.  When that happens, the best thing to do is stay home (to avoid spreading germs to others), treat your symptoms, and get rest.  This will allow the immune system to mount an effective response to the infection. In fact, some of the uncomfortable symptoms of viral infections (including fever, malaise, and headaches) result from your body’s attack on the pathogen and prove that your immune system is fighting the infection.

There are medicines (antivirals) that are available to treat certain infections. However, antibiotics WILL NOT work to treat a viral infection.  Antibiotics work by inhibiting the growth of bacteria by targeting processes specific to bacterial growth. Some antibiotics target bacterial cell walls while others target bacterial ribosomes.  Still others prevent bacterial DNA replication, transcription, or metabolite synthesis. Since viruses are acellular and can only reproduce inside an infected host cell, antibiotics are useless at treating infections caused by viruses.  In fact, taking antibiotics when you have a viral infection can lead to antibiotic-resistant bacteria.*

How Well do You Know Your Germs?

Look through this list to see if you can correctly identify which illnesses are caused by bacteria (and therefore can be treated with antibiotics) and which are caused by viruses. The answers are posted below.

A table listing common diseases that are either viral or bacterial in origin

*The Center for Disease Control (CDC) considers antibiotic-resistant bacteria “a global threat” and “one of the biggest public health challenges of our time” Antibiotic / Antimicrobial Resistance (AR / AMR)

Further Reading

Clinical Signs and Symptoms of Influenza

The Incubation Period of a Viral Infection

Viral Diseases 101

How do Cold and Flu Viruses Spread?

Content for this post was taken from the course Microbiology , now available here.

Scientifically-speaking, viruses are not living things. How then can they cause disease? Viruses exist that are able to infect humans, plants, animals, insects, fungi, amoeba, algae, and bacteria. How do viruses infect cells? How do you treat a viral infection? How well do you know your germs? Which of these diseases are caused by a virus?

2 thoughts on “The Peculiar “Life” of a Virus”

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