It is amazing, from a strictly objective viewpoint, that something as small as a virus can create such havoc in human populations. And from a strictly scientific viewpoint, viruses are fascinating, amazing things. As the 2019-2020 coronavirus pandemic spread across the globe, I began to ask myself numerous questions. What exactly is a virus? Where did they come from and how long have they been around? What can I learn about their biology? And how is it possible that they can bring human civilization to it's knees?
I’ve taken a very shallow dive into the pool of knowledge about viruses, and this post will be my attempt to provide a hopefully simple summary of what I've learned. A bit wonky, perhaps, but hopefully helpful for those who like wonk. And keep in mind that I am in no way an expert, or even someone with more than a beginning understanding of this science. So take this blog post with a grain of caution that it might not be exactly correct.
As an aside, I had the thought that my quest to understand the viral world is akin to trying to understand the physics of the universe, and that there would be a lot of stuff to read that I would not comprehend. I was correct!
A virus is not a plant or animal or bacterium; in fact, they are not living things, so my wondering about the “biology” of virus is a misnomer. They cannot self-replicate and cannot produce their own energy. So they are things, or agents, and the study of them is chemistry. (Perhaps this is an over-simplification, but I’ll go with it.)
So, let's look at a coronavirus. Basically, it is a tiny bit of genetic material, RNA, inside a capsule. It is called a varian. This particular varian is about 100 nanometers (nm) diameter. A nanometer is one one-billionth of a meter. For scale, the diameter of a human hair is about 70,000 nm, so about 700 of these varians laid side by side would equal the diameter of a human hair. Tiny.
Proteins and lipids and RNA; seems quite simple. The spikes, which make the varian resemble a crown, hence “corona,” interact with receptor proteins in the outer membrane of very specific animal cells. In the case of humans, the spikes activate when they contact cells of the respiratory system, in the throat and lungs; however, recent evidence suggests that this corona virus also attacks other cells, such as kidney, intestine, and maybe others.* Once attached, the varian combines with the human cell, either through fusion or invasion. It then uses the mechanisms of the human cell to replicate the varian RNA and expel new varians. The process is open to mutation, wherein parts of the human RNA get recombined with the varian RNA. The vast majority of these mutations appear to be very minor and harmless, in the context of human disease; however, there are exceptions. This process is shown in the next figure, and if you understand all the notations, explain them to me! Basically, the figure shows a varian attached to a human cell, top left, the processes that occur within the cytoplasm of the cell that replicate the varian RNA, and a new varian being expelled from the cell, top right.
The illness caused by the invasion of this coronavirus is COVID-19 (COrona VIrus Disease 2019). It is typically manifested in three stages in those individuals who get critically ill. It is important to note that, epidemiologically, a large proportion of people who are infected with this virus do not exhibit symptoms and, if they do get ill, have a mild case of the disease.
In Stage 1 of COVID-19, the virus kills the cells it has invaded, and these dead cells slough off and start to clog the lung airways, causing pneumonia. In Stage 2, the body’s immune system rallies to attack the virus and becomes over-active due to the severity of the invasion, and the result is that the immune system starts to kill lung cells. More dead cells are sloughed off, worsening the pneumonia, and as this progresses, it basically eats holes in the lung tissue and scar tissue forms, which stiffens the lungs; this is Stage 3. The result can be respiratory failure, which is why so many critical patients are intubated and put on a ventilator. Changes to lung cell membrane permeability cause the lungs to fill with fluid. As a result of all the above, the lungs can fail to infuse enough oxygen into the patient’s blood, resulting in death.
Sounds pretty gruesome, because it is.
Where did viruses come from, and when? My surface scratch research suggests that there are several theories about the origins of viruses. One theory is that viruses appeared prior to single-cell life forms, and may or may not be linked to the evolution of living organisms. Another theory posits that viruses appeared within the same time frame as living cells.
In my simple view, however they originated, viruses developed to take advantage of living organisms by using the organism’s cells to do the work of replication, thereby keeping the varian more “simple” by not needing feeding, reproductive and other systems. I do not think that viruses are simple; they are very complex chemical packets.
Health experts expect this virus to be cycling through the population for more than this cycle. There are some estimates that outbreaks will occur next fall/winter that could be as bad as the current outbreak. In other words, this is not a short-term event.
—-
(This post was modified on May 11, 2020 by adding the footnote.)
* There are a lot of unknowns concerning this particular coronavirus, and new information about it is coming out of research labs and groups almost daily. Recent cases have indicated that COVID-19 might be related to strokes in otherwise healthy people in their 30's and 40's. In New York, a number of children who were infected but asymptomatic have developed a mysterious illness, and at least one child has died.
No comments:
Post a Comment