What is a virus?
A pathogen is a biological agent that causes disease or illness in a host. Viruses are the most abundant pathogen on Earth. Viruses are microscopic parasites, generally much smaller than bacteria. They lack the capacity to thrive and reproduce outside of a host body such as bacteria, animals, or plants.
Each viral particle, or virion, consists of a single nucleic acid - RNA or DNA - surrounded by a protein coat.
The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope as they are one hundredth the size of most bacteria.
Outside of a cell, a viral particle is inert. A virus is capable of replication only within the living cells of a host. Once a host cell is infected, the host is forced to rapidly reproduce thousands of identical copies of the virus, which are then able to spread and infect other cells.
Ebola virus, 1976. Courtesy of the CDC
Is a virus alive?
Viruses are not capable of reproducing on their own; they can only reproduce within the living cells of a host organism. While many people debate the definition of what makes something alive, the ability to reproduce independently is often considered to be one of the seven characteristics of living things. Viruses are genetic material (DNA or RNA) but are not generally considered to be alive.
Where are viruses found?
Wherever there is life, there are viruses. They are found in soil, the air that we breathe, and oceans. They are a part of almost every ecosystem on Earth. They even live in extreme environments such as hot springs, deep ocean thermal vents, and Antarctic ice.
Are all viruses bad?
Not all viruses are harmful. Viruses play a critical part in our ecosystem and are helpful in sustaining life on Earth. For example, viruses help the microbes in the ocean produce oxygen. The oxygen that these microbes produce accounts for nearly half of the oxygen on the planet. A class of viruses known as bacteriophages can even kill a spectrum of harmful bacteria, providing protection to humans.
How do viruses spread?
Viruses spread in many ways.
One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects, such as mosquitos.
Infection can also occur via the exchange of blood and bodily fluids, via contaminated food or water, respiration of viruses contained in aerosols, and fecal matter. Some viruses can live on surfaces outside the human body and can be acquired through cross-contamination due to unsafe handling practices.
Some viruses are spread easier than others. The term R0 (pronounced “R naught”) is used to describe the transmissibility of a disease. Each person infected with a disease with an R0 of 5 would, on average, infect 5 otherwise healthy people. The R0 of the 1918 Swine flu (Spanish Flu) was estimated to be between 1.4 and 2.8.
What infections do viruses cause?
Viruses infect all living organisms: plants, animals (including humans), and bacteria. Only a tiny fraction of the viruses that surround us actually pose any threat to human life or health. Some well-known human diseases caused by viruses include influenza, chickenpox, HIV, Ebola, cold sores, and the common cold.
The term “Virulence” is defined as the ability of a microbe to cause disease or damage in a host. In other words, the more virulent a virus is, the more dangerous it is to its host.
How does the human body fight against viruses?
Viruses make us sick by killing cells or disrupting cell function.
Our bodies seek to identify infections in several ways, and once identified will try to fight the infection. This fight, known as the immune response, includes the secretion of a chemical called interferon (which blocks viruses from reproducing), or the production of antibodies, and more.
In response to an infection, your immune system springs into action to destroy the disease. Many of the symptoms we feel when sick - fever, malaise, rash, headache - are a result of our own immune system trying to protect us.
Drugs prescribed by a doctor can also help your body to fight infections. People will often receive a prescription for an antibiotic such as Amoxicillin or Azithromycin as a treatment for a disease caused by a bacteria. However, antibiotics don’t work for infections caused by a virus.
Protecting yourself from disease
• Practice good hygiene: wash your hands thoroughly or use hand sanitizers
• Ensure that your food and drinks are safe to consume and hygienically prepared
• Cover your mouth and nose with a tissue when you sneeze or cough
• Practice safe sex and get tested for HIV and other sexually transmitted diseases
• Use insect repellents to avoid bug bites and limit outdoor activity during peak mosquito hours of early morning and evening
• Stay clear of wild animals. Many wild animals, including raccoons, skunks, foxes, and coyotes, can spread rabies to humans by biting. Keep your pets away from wild animals as well, and make sure their vaccinations are up to date
• Get vaccinated to protect yourself and others
• Take travel precautions: follow the guidance of the World Health Organization and Government of Canada when travelling and avoid travelling to places where travel is not recommended or where there are Travel Health Notices. Work with your doctor to get additional immunizations or travel medicine as recommended.
How to protect society from disease
The velocity of the spread of disease correlates with a number of factors: human population density and sanitation being two of the most significant. There is evidence that many highly lethal viruses lie in biotic reservoirs in remote areas, and epidemics are sometimes triggered when humans encroach on natural areas that may have been isolated for long time frames; this theory has been suggested for many dangerous diseases including Ebola, HIV, and COVID-19.
Herd Immunity
Herd immunity is a term used to describe what happens when a large portion of a community has become immune to a disease. When enough people in a community have immunity to a disease, it makes the transmission of the disease very unlikely. The percentage of a population that must be immune to achieve herd immunity is called a “threshold proportion.” This percentage depends on how contagious the disease is - the R0. For some very contagious diseases, such as measles, this threshold can be as high as 94% of the population. To achieved herd immunity, enough people need to receive a vaccine for a disease or suffer from and survive a disease to build natural immunity.
Resources for Additional Reading
Viruses:
Infections:
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