You have come across them at some point in your life. These creatures have also been trouble for humans. Not just humans, but for animals and plants too. Viruses are such creatures that don’t care what species you belong to, all they wanna do is reproduce. But these viruses have perplexed humans ever since they were being discovered. And a question that often comes into everyone’s minds is; Is a virus a cell or not? What are they? Are they living or dead? And the most interesting question, can viruses get sick?
What is a Virus?
It is difficult for me to categorize viruses into any category as it is also difficult for virologists and researchers to do so. Viruses sit on the fence on being living and dead. Some aspects of viruses indicate that they are living, but some are so perplexing that they look non-living. Maybe viruses are something in between? Maybe they shatter the dividing wall between living and dead. Or maybe when you ask a virus, are you dead or alive, they say “yes”, who knows.
In scientific language, a virus is a piece of single or double-stranded DNA or RNA that is enclosed in a protective protein cover called a capsid. This DNA has some enzymes with it to help it infect other cells and make more copies of itself. This inability of the DNA or RNA to make more copies of itself independently what makes the virus non-living. But then when it gets into a host cell, it starts reproducing. This makes the virus living. Confusing right? There’s more.
Is Virus a Cell
A question that has just four words, but the answer for this would need more than four hundred. First, what does a cell mean? A cell is an enclosed space with genetic material and organelles that perform certain functions. This is what cells are. When we look at a virus, it has genetic material but it is not enclosed within a membrane (nuclear membrane). Viruses also lack organelles that are essential for life processes. They do have a protein enclosure. So for the question is a virus a cell, the answer would be yes and no, a quality that virus always shows.
Organelles are the components that are lacking in a virus. It would be correct to say that viruses are obligate parasites. Like a leech has lost its digestive system and relies on other organisms’ blood to survive, viruses are like parasitic DNA that needs a host to become living. All known viruses are single-celled which means they are never found in a cluster till date. There is one small exception which will be discussed later in the article.
The Discovery of Viruses
There are a lot of great articles about the discovery of viruses. I will not get into detail about that as that would be digressing. But I’ll give the barebones info in a palatable form.
Scientists were puzzled at one plant disease that seemed to be caused by an organism that was not bacteria. This disease was the Tobacco Mosaic disease of Tobacco plants. A scientist named Adolf Mayer wanted to find the causative organism.
He took some leaves of an infected plant and crushed it, making it a thick paste and then extracting the water from it. This water was injected into a healthy plant. The healthy plant soon started showing the symptoms of the disease. It was confirmed that the organism was in the water, but could not be seen under the microscope. The mystery intensified. He also tried different infecting the plant with bacteria but none could bring the effect.
Another scientist named Dimitri Ivanosky repeated the experiment but he added one extra step. He filtered the infected water with a Chamberland filter. This would remove all the known bacteria. The water still caused the disease. Something had passed through the filter.
While Ivanosky said that it could be bacterial toxin causing the disease, the correct hypothesis with the wrong name was suggested by Martinus Beijerinck in 1898. He said that the disease was caused by a filterable virus. Also in 1898, Friedrich Loeffler and Paul e found that Foot and Mouth disease in livestock was caused by something smaller than bacteria.
Why did no one see the Virus?
There is a reason why these sneaky viruses went undetected from the scrutinizing eyes of the scientists. Viruses are extremely small, so small that even bacteria are large in comparison with them. If a virus (Measles virus) was the size of humans, let’s say 6 feet tall, the E. coli bacteria would be as tall as the Hollywood Sign. That’s tall. You can see the different sizes of cells here.
Another example would be the Hepatitis virus. If the virus was 6 feet tall, the E. coli bacteria would stand as tall as the shoulders of the Statue of Liberty. Now you can wonder why these viruses kept passing through the filters. And these viruses are sneaky, causing some of the worst human diseases. Here are some examples;
The Ebola virus caused the 2014 Ebola endemic. The H1N1 virus caused the Swine Flu pandemic. The recent SARS-CoV-2 which is causing the COVID-19 pandemic. Other diseases include hepatitis, HIV-AIDS, Polio, Flu, Common Cold, SARS, MERS, Herpes, Dengue, Yellow fever, Chickenpox, smallpox, etc. The list goes on. This small, living/non-living piece of DNA/RNA has killed a lot of humans. Let’s see how these viruses get inside our cells.
Viruses have a Get Inside Pass.
The virus is a genetic parasite. As it lacks the organelles needed for protein synthesis and life, it needs a host. Living cells have a lot of complicated mechanisms that help them maintain homeostasis. A virus is just a piece of DNA/RNA until it gets inside the living cell. Then it makes the host cell zombie. So Virus us a parasite that can make a cell into a zombie. But how does a virus gets inside the cells? It has a pass.
Normal cells have a cell membrane that acts as a gate. This gate allows some proteins inside and all other useless things are rejected. To recognize which proteins to let in, the membrane has a receptor protein. When a useful protein touches the receptors (like lock and key), the shape of the protein matches the receptors and the gate opens.
Viruses replicate this shape as if making a duplicate key. This is how the enter inside healthy cells. But the process does not stop there. After the virus has bypassed the security check, it chooses how to enter the cell. Different viruses have different ways of getting in.
The HIV that causes AIDS attaches itself to the cell and then injects the RNA (genetic material) into the cell along with the enzymes. The RNA then hijacks the nucleus of the cell and turns it into a virus factory.
Poliovirus bores a hole in the cell and gets inside while Flu virus goes inside the cell completely. And what happens after that, you are aware of it.
Once a virus gets inside the cell and infects it, it is then called a virion. The capsid (which is the covering of the virus) is there to protect the viral DNA/RNA from digestive enzymes.
The Phases of Viruses
Is a virus a cell might be a difficult question to answer, but this new property of viruses makes it even more difficult. Viruses are known to have two different phases, the lytic phase, and the lysogenic phase.
In the lytic phase, the virus reproduces actively, using the resources of the host cell. This can also be an attribute of a cell. Like cells replicate, viruses show this similarity. So is a virus a cell? This property shows it is.
In the lysogenic phase, the virus lays dormant and does not actively reproduce. As the host cell multiplies, the virus also multiplies passively. This is not an attribute of any cell. So is a virus a cell? This shows that it isn’t. See the problem?
The Origin of Virus
There aren’t any solid, proven theories about the origin of viruses. We even don’t have a proven theory of the origin of normal life. But some hypotheses try to give some idea about it.
The Progressive Hypothesis suggests that some genetic material from some cells started moving out of the nucleus along with some enzymes. They got separated and started living life as parasites.
The Regressive Hypothesis suggests that some cells got tired of the difficulties of living life and started shedding off their complex organelles until just the DNA/RNA remained (this is not the language used in the scientific papers).
Koonin and Martin in 2005 suggested that maybe viruses were the first living/non-living organisms that evolved, way before normal living cells. This is called the Virus-first hypothesis. We don’t know how the virus came into the world, but we do know one place where they will stay as long as humans live; In Our DNA.
The Virus are in Our DNA
The Human Genome Project is an ambitious project that is undertaken to sequence and records the entire human genome. But when we analyzed the DNA of humans, we found that about 8-10% of our own DNA had foreign origins. Called the endogenous retrovirus, these DNA sequences are from viruses that decided to permanently take residence in our DNA and stop this process of infecting. You can read more about it here.
The Problem of Treating Viruses
Viruses are fast-evolving organisms as they reproduce so quickly. This creates a huge problem in treating the diseases caused by them. For normal bacteria, antibiotics work fine, but viruses need something drastic.
Suppose we get a drug that kills almost 99% of the virus inside an infected body. The 1 percent then multiplies quickly with some genetic mutation that makes the drug ineffective against them. Getting a new drug that kills 99% or fewer repeats the previous situation. This is why AIDS is such a dangerous disease and this is why you need flu shots every year.
Can Viruses Get Sick?
This is a very perplexing question but it can happen. Microorganisms can also get infected by other microorganisms. It’s like robbery in the house of a robber. So how does that happen? And is a virus a cell if it can get infected as other cells do? Let’s see.
Scientists discovered a new type of viruses that are really large. These are called Mimivirus, Mamavirus, and Megavirus
In the case of Mamavirus, there is a small virus that lives inside this large virus, called Sputnik. Mamaviruses infect amoeba cells and take over them to produce more mamaviruses. But here the smaller virus interferes. Once the mamavirus has taken over, the sputnik hijacks the amoeba and takes control, infecting the larger Mamavirus. Poor little amoeba has to go through two parasites.
Maybe answering is a virus a cell is not that easy, but why should we call it a cell? Viruses are something more than living or dead, they are both. Maybe we need a new classification of life for them, a life that lives when the environment is favorable and hibernates when it is not. What do you think? Is a virus a cell? What are your thoughts?