With the Corona Virus in pandemic mode, time to discuss the nature of viruses in general and how they're governed by Natural Selection.
First, what is a virus? Relatively simple to define, but difficult to explain. Because viruses straddle the boundary between "living" and "non-living," they belong to a special class of organisms (if you want to call them that) that don't fit into biological models of life on Earth.
A virus doesn't carry on life functions - they're essentially a DNA or RNA molecule wrapped in a protein coat. Additionally, viruses are wholly dependent on a host, and I as used to tell my students, they're genetic parasites.
A virus in independent state is known as a virion, and probably fits the definition of non-living. In fact, they can be crystallized and stored like a chemical compound.
It's only when they infect a host do they seem to "come alive." But in fact, they're simply manipulating a host's DNA or RNA to make more copies of itself.
Another discussion I used to have with students, started with the statement: "All viruses cause disease." Without exception. And they infect every known species of life on Earth, and yes even bacteria.
And they've evolved extraordinary strategies to spread themselves. Think of the common cold - a rhino virus usually. One gets a cold, what happens? the virus has infected the mucous membrane, taking over cells, causing them to reproduce millions of copies of the virus. The body wants to rid itself of the virus, so it coughs and sneezes sending those virions out into the environment so they can find the next host.
Since they infect respiratory surfaces, rhino viruses spread rapidly through the population because of mechanisms in place to rid the body of the disease. Rather ironic. Our personal "cure" involves potentially infecting others...
A pandemic? Not difficult to understand when we consider viruses that infect the respiratory system. With a world population over 7 billion and modern travel, respiratory viruses have an enormous surface to spread rapidly.
Yet rhino viruses do not cause pandemics. We recover from colds. Pandemics occur when certain types of viruses, influenza and now Covid-19, spread and induce a high mortality rate and increased hospitalization.
Which brings me to Natural Selection, one of Charles Darwin's great contribution to biology. The power of his theory is evident in that he did not know of viruses, yet they behave in a manner consistent with his theory.
Generally, within any large population, their exists enormous variation. Certainly true for humans, all life, and yes, viruses too.
When viruses infect, they cause their host to make copies. Since this is genetic replication, mistakes are made, also known as a mutation. In essence, a variant of the original virus has been created.
This variant may be benign, or in the case of the H1N1 influenza virus, the new strain is deadly to its host and infects at a higher rate, all because of a genetic mutation. With a higher infection rate, and an infection that's toxic to human organs, a pandemic may occur. This happened with the H1N1 influenza virus in 1968, 1918, and 1890.
Fortunately, we have a dynamic immune system, and just as importantly, and as a population, subject to natural selection as well.
As the virus spreads, Natural Selection will favor those with a rapid immune response, or have no underlying health issues. After the wave of infection, the population in general will have "adapted."
Of course, since viruses also adapt, a new strain will appear possibly creating the next pandemic. It's interesting to note though pandemics occur every 20 to 50 years.
It's an ongoing biological clash between infectious agents and their hosts. Will we survive? Of course we will. Besides an adaptive immune response, Natural Selection has given humans the capacity to intellectually understand and socially adapt, creating an environment to limit the infection rate, as we're doing now.