How they work
Viruses are in an inactive phase when left outside a cell. They seem dead during this phase. When they find a cell, however, they become active. They "hijack" the cell's functions, and make it replicate the virus' DNA instead of the cell's. This behaviour is very complicated, but it can be prevented by our natural immune systems.
Instead of the cell reproducing, it produces copies of the virus itself using the genetic material provided by the virus. After the copies of the virus have taken over the entire cell, the cell membrane bursts open. This helps to infect new hosts, to reproduce more.
They're not all bad, though!
Some viruses are used to strengthen crops, by increasing disease resistance. Others, such as HIV, are used in curing certain types of cancers.
What is the most remarkable of all, is that many scientists believe some viruses that infected bacteria billions of years ago helped form the first cell nucleus. If this is true, then all complex cellular life on Earth descended from a single virus and its attempt at trying to infect a bacterium!
Are they alive?
Viruses are thought of as being in a gray area between living and nonliving. They cannot replicate on their own, but can do so in truly living cells and can also affect the behavior of their hosts profoundly.
The categorization of viruses as nonliving during much of the modern era of biological science has had an unintended consequence: it has led most researchers to ignore viruses in the study of evolution.
Viruses, parasitize essentially all biomolecular aspects of life. That is, they depend on the host cell for the raw materials and energy necessary for nucleic acid synthesis, protein synthesis, and all other biochemical activities that allow the virus to multiply and spread.
One might then conclude that even though these processes come under viral direction, viruses are simply nonliving parasites of living metabolic systems. But a spectrum may exist between what is certainly alive and what is not.
A rock is not alive. A metabolically active sack, devoid of genetic material, is also not alive. A bacterium, though, is alive. Although it is a single cell, it can generate energy and the molecules needed to sustain itself, and it can reproduce.
But what about a seed? A seed might not be considered alive. Yet it has a potential for life, and it may be destroyed. In this regard, viruses resemble seeds more than they do live cells. They have a certain potential, which can be snuffed out, but they do not attain the more autonomous state of life.
How viruses jump from animals to humans
This is, of course, seen in the recent Covid-19 pandemic. The virus was transferred from bats to humans. How did this happen?
To survive and reproduce, viruses must move through three stages: contact with a susceptible host, infection and replication, and transmission to other individuals.
Viruses are constantly encountering new species and attempting to infect them. More often than not, this ends in failure. This is because in most cases, the genetic dissimilarity between the two hosts is too great.
There are a staggering number of viruses circulating in the environment, all with the potential to encounter new hosts. Because viruses rapidly reproduce by the millions, they can quickly develop random mutations. A small proportion of these random mutations may enable the pathogen to better infect a new species. If the new species is closely related to the virus’ usual host, the odds of winning this destructive genetic lottery increase.
Once a host jump reaches the transmission stage, the virus becomes much more dangerous. Now gestating within two hosts, the pathogen has twice the odds of mutating into a more successful virus. Each new host increases the potential for a full-blown epidemic.
Virologists are constantly looking for mutations that might make viruses such as influenza and Covid-19 more likely to jump. However, predicting the next potential epidemic is a major challenge.
The evolution of viruses
Viruses have their own, ancient evolutionary history, dating to the very origin of cellular life. For example, some viral-repair enzymes which excise and resynthesize damaged DNA, mend oxygen radical damage, and so on, are unique to certain viruses and have existed almost unchanged probably for billions of years.
Viruses matter to life. They are the constantly changing boundary between the worlds of biology and biochemistry. As we continue to unravel the genomes of more and more organisms, the contributions from this dynamic and ancient gene pool become more apparent.
Regardless of whether or not we consider viruses to be alive, it is time to acknowledge and study them in their natural context - within the web of life.