Sunday, March 29, 2020

An Introduction to Viruses

coronavirus influenzaI’ve heard it said that there are ten million times more viruses on Earth than there are stars in the universe; maybe more. And that scientists estimate that, at any given moment, there are more than a billion viruses present on earth.

Viruses are smaller than bacteria. For example, at 45 nanometers (1 nanometer = 0.000001 millimeter), the hepatitis virus is approximately 40 times smaller than E.coli. By comparison, a human red blood cell is around 6,000 to 8,000 nanometers in diameter.

Bacteria are single-celled microorganisms, most of which are harmless to people. Some are even beneficial (e.g. Lactobacillus acidophilus, which is found in yogurt and, after digestion, resides in the intestines, participates in the digestion process, and aids in preventing the growth of harmful bacteria). Others are not, and cause diseases including strep throat, urinary tract infections, and tuberculosis. But perhaps the most important distinction between bacteria and viruses is that, unlike bacteria, antibiotic drugs are ineffective against viruses.

What is a virus?

The word virus comes from the Latin ‘virus’, meaning poison or noxious substance. Simply stated, viruses are any of a large group of sub-microscopic infectious agents, which produce a wide range of significant diseases in humans, animals, and plants.

Viruses consist of genetic material; either DNA or RNA; surrounded by a protective protein coating, or shell, called a capsid. They’re generally believed to be non-living however, because they don’t possess the biochemical mechanisms needed for replication. Instead, they replicate by using the biochemical mechanisms of a host cell.

Outside of a host cell, viruses don’t use any energy. But, when a virus particle, or virion, comes into contact with a host, the virus becomes active; injecting its viral nucleic acid (and sometimes a few enzymes) into the host cell. The virion then uses the host cell’s energy and tools to create more viruses.

Viral diseases in humans include influenza (flu), measles, chicken pox, herpes, small pox, HIV/AIDS, leukemia, and Ebola. Viral diseases that impact animals include rabies, foot-and-mouth disease, swine flu, bird flu,distemper, and equine encephalitis.

Plant viral diseases include mosaic virus (which commonly infects tomatoes, melons, squashes, cucumbers, and tobacco), leaf roll and leaf curl viruses (which infect tomato, pepper, eggplant, potato, beans, melons, pumpkins, squash, tobacco, and other crops as secondary hosts) and ringspot virus (which infects tomato, raspberry, hops, roses, and tobacco).

By helping the body to build an immune system response against specific viruses, vaccines have been effective at preventing some types of human viral infections.

Identifying a virus for the first time

In 1886, Adolph Mayer, director of the Agricultural Experiment Station in Wageningen, Holland published a research paper titled, ‘Concerning the Mosaic Disease of Tobacco’, in which he noted that ‘tobacco mosaic disease’ could be transferred between plants by crushing infected leaves and injecting the extracted juice into the veins of healthy tobacco leaves. Although he was confident that the disease was spread by bacteria, he was unable to isolate the disease-causing agent, identify it using an optical microscope, or recreate it by injecting healthy plants with any known bacteria.

In 1898, Dutch scientist Martinus Beijerinck proposed that tobacco mosaic disease was caused, not by bacteria, but by a poison; a ‘filterable virus’. But it wasn’t until 1939 that Ernst Ruska, a German physicist and Max Knoll, a German electrical engineer, using the recently-developed high-resolution technology of the electron microscope, which they’d invented in 1931, that scientists were actually able to see, identify, and micrograph the tobacco mosaic virus.


Coronavirus get their name because of their crown-like shape. Although they were first identified in the 1960s, we still don’t know where they come from. According to the World Health Organization, coronaviruses are a large family of viruses that cause illness ranging from the common cold to more severe diseases, such as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS). Coronaviruses are zoonotic, meaning they are transmitted between animals and people. Detailed investigations found that SARS was transmitted from civet cats to humans and MERS from dromedary camels to humans. Several known coronaviruses circulating in animals have not yet infected people (e.g. feline coronavirus).

Coronaviruses typically travel through the air in tiny droplets that are produced when a sick person coughs, sneezes, talks or breathes. Those droplets fall to the ground within about six feet.


COVID-19 is a new strain of coronavirus that was discovered in 2019 and had not been previously identified in humans. It’s part of the larger family of coronaviruses.

Common signs of infection include respiratory symptoms, fever, cough, shortness of breath, and breathing difficulties. Symptoms typically go away in a few days. But if a coronavirus infection spreads to the lower respiratory tract (your windpipe and your lungs), it can cause pneumonia, especially in older people, people with heart disease, or people with weakened immune systems. Infection may also cause severe acute respiratory syndrome, kidney failure, and death.

Standard recommendations to prevent infection-spread include regular hand washing, covering your mouth and nose when coughing or sneezing, and avoiding close contact with anyone who is coughing, sneezing, or showing any other symptoms of respiratory illness.

There is now widespread community transmission of COVID-19, meaning that sources of new infections are unknown. Experts estimate that the incubation period is somewhere between two and eleven days although, at the time of this writing, they have yet to determine if the new coronavirus can be caught from individuals who aren’t symptomatic. Therefore, the federal government is stating that you need to isolate yourself and self-monitor at home for 14 days, even if you don’t have symptoms.

Illustration: Left: What COVID-19 coronavirus looks like under a microscope. Right: What influenza virus looks like. Courtesy Centers for Disease Control (CDC).

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Richard Gast is a retired Extension Program Educator and has been contracted by Cornell Cooperative Extension Franklin County to continue his informative and thought provoking articles.

6 Responses

  1. Nora says:

    Thank you Mr. Richard Gast for a very well written informative and easy to understand report on the coronavirus.

  2. Boreas says:


    “And that scientists estimate that, at any given moment, there are more than a billion viruses present on earth.”

    Is this a typo? Seems pretty low, given there are many more humans than that. Did you mean a billion varieties?

  3. Richard says:


    Absolutely excellent article–great context and super-efficient summary of information that is so basic but so lost in the immediate conversation right now. Another great piece.


    Richard Halpin

  4. James Marco says:

    Boreas, no typo, in fact, it is likely a bit low.
    Humans, have no immunity to it. After looking at close to a million cases, no natural immunity has been found. There are a wide range of illness caused by COVID19, from very sick/terminal to very mild. The stronger your immune system the stronger you will resist corona-virus. Exercise, .balanced diet, previous exposure to related diseases, loss of excess weight, can help somewhat, but the effect is not strong. Older people, and those with weakened immune systems are VERY susceptible, nothing new here. Infants develop a good immune system by 3-6mos. This is a fairly good general article on an aging immune system (>70 years.)

    There are three types of immunity. Natural immunity basically means you will mot catch a disease. This has not been found with the corona-virus. Of the two remaining, one is acquired, usually through catching a disease or through a vaccine, and one is temporary, usually by transfusion/refining/manufacturing antibodies that will last a few weeks to several years in the bloodstream. (I am sure our glorious leaders are using this technique.)

    Until an effective vaccine is developed, COVID19 will effect everyone, eventually. All we can do is delay it, develop techniques for dealing with the infection, hope like hell we don’t overload the medical facilities, and, keep our medical personnel going (preferably in shifts as they too catch it and get over it…it will be with us for a long time.)

    • Boreas says:


      That is what I am saying – not only a bit low but WAY low. There are always billions of bacteria in/on every healthy animal, let alone plants. Obviously, bacteria are not viruses, but there may be a billion viruses of just one variety in each severely infected individual, let alone “…at any given time on earth.”. That is why I was asking for clarification on that statement.

  5. Luc Poirier says:

    Dear Richard, I am glad to read article like yours base on medical and scientific facts and not on feelings and believes. Hope to talk about it with you soon.

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