What Is A Virus And Which Viruses Are The Most Deadly?

By Rick Gonzales

What is a virus?

Welcome to the new norm. How long this new norm will last is anyone’s guess. The experts certainly don’t know because they are still trying to determine the best way to combat COVID-19, the new coronavirus. While they are doing this, why don’t we take a bit of time to answer this question: What is a virus?


A virus is a microscopic infectious organism that cannot replicate by itself. A virus can only reproduce inside the living cells of an organism.  Most viruses are so small that they can only be seen with at least a conventional optical microscope. Viruses have the ability to infect a large variety of living organisms that include plants, animals, archaea, and bacteria.


When a virus invades living cells, they use their chemical machinery to keep itself alive. This is when they can replicate. They then can reproduce by entering the host cells and more or less hijack enzymes and materials of the host cells, making more copies of themselves. They can make exact copies or they can create mutations. The ability to mutate allows some viruses to change slightly in each infected person, many times making treatment difficult.


Sometimes a virus doesn’t immediately strike a living cell. So, while they are not in the process of infecting a cell or not already inside an infected cell, they are a form of virions (independent particles). This consists of a) genetic material (long molecules of RNA or DNA responsible for the structure of the proteins in which the virus acts; b) the capsid (a protein coat that surrounds and protects said genetic material; c) a lipid envelope is, in some cases, present around the capsid.


A DNA virus has DNA as its genetic material. A DNA-dependent DNA polymerase (enzyme) is used for this virus to replicate. An RNA virus differs as it uses RNA (ribonucleic acid) as its genetic material.

David Baltimore developed a virus classification system (Baltimore classification) that took viruses and grouped them into families based on their type of genome. DNA viruses are in Groups I and II. RNA viruses belong in Groups III, IV, and V.


As far as biological entities go, viruses are by far the most abundant on Earth. In fact, they outnumber all other biological entities put together. They run the gambit in terms of the cellular life they infect. There are different types of viruses that infect only a limited range of hosts, many of those are species-specific. For instance, the smallpox virus is one that infects only one species – humans. It has a narrow host range. Other viruses have the ability to infect different species of mammals. One example would be the rabies virus. Most (we’ll stress MOST) viruses that infect animals are harmless to humans. Viruses that infect plants are safe for animals.

As far as the exact number of viruses, that number has never been determined. In a study published by the American Society for Microbiology in 2013, they estimated nearly 320,000 types of viruses infect mammals alone. According to professor and virologist Curtis Suttle with the Institute for the Oceans and Fisheries at the University of British Columbia, “Every day, more than 800 million viruses are deposited per square meter above the planetary boundary layer — that’s 25 viruses for each person in Canada.” Given what is going on in the world today, that is a scary thought.


The only reason a virus exists is so it can reproduce. They love new cells and new hosts. The more the merrier. And it all depends on the virus’s makeup for its ability to spread. Viruses can be transmitted from person to person. They can also be transmitted from a mother to their child during pregnancy or delivery.

There are some viruses that have the ability to live on an object for a length of time. If a person with a virus touches an item with the virus on their hand, another person can pick up that virus by touching the same object. These objects are called fomites. There are many examples of fomites which can carry a virus. Clothes, utensils, doorknobs, and furniture are just some. In a hospital, which shouldn’t be surprising, some common sources of fomites are skin cells, hair, and bedding.

Viruses can also be spread by droplets, typically when a person sneezes near another person or coughs. The droplets can be breathed in by a person or the droplets can land on an object that another person touches.

Once the virus takes hold, it will begin to replicate in the body and affect its host. The incubation period starts and after this, the host will begin to show symptoms.


H1N1 Flu Virus

When asking what is a virus, you also will want to know what the most comment types are. With viruses numbering in the hundreds of thousands, there are some common ones.

COMMON COLD – This one affects an adult perhaps two to four times a year, kids possibly a little more. The common cold is caused by a couple of viruses; an adenovirus, a coronavirus, or a rhinovirus.

INFLUENZA – This is the virus that causes the flu and there are many strains. This virus mutates from year to year, so a cure is hard to come by. It is suggested to get your flu shot every year to help prevent the flu as worldwide it is estimated that between 250,000 and half a million people die each year from the flu.

BRONCHITIS – This can be caused by a virus, bacteria or even chemicals but the viral version is the most common. This virus can cause a cough that can last for weeks and can be a complication from both cold and flu.

GASTROENTERITIS – This is the very popular stomach flu, which is a common viral infection. It is very unpleasant, causing vomiting and diarrhea and is highly contagious.

CORONAVIRUSES – Human coronaviruses were first identified in the 1960s. They have been put into four main sub-groups known as alpha, beta, gamma, and delta. In these sub-groups, there are seven identified coronaviruses that can infect humans. The Common Human Coronaviruses are:

  • 229E (alpha coronavirus)
  • NL63 (alpha coronavirus)
  • OC43 (beta coronavirus)
  • HKU1 (beta coronavirus)

Then we have Other Human Coronaviruses. They are:

  •  MERS-CoV (the beta coronavirus that causes Middle East Respiratory Syndrome, or MERS)
  •  SARS-CoV (the beta coronavirus that causes a severe acute respiratory syndrome, or SARS)
  •  SARS-CoV-2 (the novel coronavirus that causes coronavirus disease 2019, or COVID-19)

Around the world, people are commonly infected with human coronaviruses 229E, NL63, OC43, and HKU1. There are times that certain coronaviruses that infect animals evolve and then make people sick. These become new human coronaviruses. The three recent examples of this are 2019-nCoV, SARS-CoV, and MERS-CoV.


Rabies Virus
Rabies Virus

Now that you know what a virus is, it’s time to figure out which ones are the worst. As if things weren’t bleak enough, right? Here are some of the world’s deadliest viruses.

Marburg virus – Imported monkeys started this one. This virus was identified in Germany in 1967 when small outbreaks among lab workers were exposed to infected monkeys imported from Uganda. This virus causes hemorrhagic fever, which is a high fever and bleeding throughout the body. This will lead to organ failure, shock, and death.

The first outbreak saw a 25% mortality rate, but it was more than 80% during the 1998-2000 outbreak in the Democratic Republic of Congo. The 2005 outbreak in Angola also saw an 80% mortality rate.

Ebola virus – The Republic of the Sudan and the Democratic Republic of Congo both saw this outbreak of Ebola simultaneously in 1976. The known strains of the Ebola vary widely in their deadliness. The Ebola Reston doesn’t even make people sick. The Ebola Bundibugyo the death rate is 50%. The Sudan strain of the Ebola is up to 71%.

Rabies – Yes, there is a vaccine for pets, but this virus is nevertheless deadly. It has become a rare disease in the developed world, but rabies remains a serious issue in India and parts of Africa. Elke Muhlberger, associate professor of microbiology at Boston University, told Live Science, “It destroys the brain, it’s a really, really bad disease. We have a vaccine against rabies, and we have antibodies that work against rabies, so if someone gets bitten by a rabid animal we can treat this person.” But she continued on, “If you don’t get treatment, there’s a 100% possibility you will die.”

HIV – In our modern world, HIV may still be the bigger killer. Dr. Amesh Adalja, the spokesman for the Infectious Disease Society of America and an infectious disease physician, said to Live Science, “It is still the one that is the biggest killer.” And she’s right. Since the early 1980’s when the disease was first recognized an estimated 32 million have died from it. The infectious disease that takes the biggest toll on mankind right now is HIV,” Adalja added. New, powerful antiviral drugs have been used to combat the disease, allowing people to live for years.


There are others we worry about. The Hantavirus, Influenza, and Dengue are among them. Smallpox was another and in 1980 the World Health Assembly declared the world free from smallpox. The vaccine worked. This is how most viruses are treated. Through medicine. Vaccines. Doctors and researchers continue the daily fight to combat these deadly viruses. But as we have been living through lately, simply being safe with one’s hygiene can help prevent an outbreak. Cover that cough or sneeze, wash your hands thoroughly. Keep your hands away from your face. Being smart will go along way in most instances.