Imagine an alien creature floating in space. It has no home and cannot move and grow on its own, but as soon as it enters a planet that suits it, it will quickly multiply and take over the entire planet in no time. .
It sounds like science fiction, but you just replace “planet” with “host,” a description that basically applies to one of the microbes around us – viruses.
Viruses are made up of two parts, a protein coat on the outside and DNA or RNA, the genetic material, on the inside. Strictly speaking, a virus is not life, because it does not have the ability to survive independently, and it is nothing without a host. But it is the most abundant organism on earth. From the ocean and forest to the livestock and people around you, to your own body surface and body, it is everywhere. This world of viruses is called the “viral circle”. Given that we know less about viruses than any other life form, the virosphere has long been a mysterious world to us.
But that is now changing. Our ability to detect viruses has increased exponentially, and the number of virus species we have identified has increased 20-fold over the past five years. What’s more, our perception of viruses is also changing: These strangely diverse microbes aren’t exactly killing machines, they also played a major role in the evolution of life on Earth.
virus planet
In terms of numbers, no organism can match a virus. Our planet is literally a “virus planet”. One study estimates that the total number of viruses on Earth reaches 10^31. Viruses are generally on the order of 100 nanometers in size. If you put all these viruses end-to-end, it’s almost 100 million light-years long, far beyond the confines of the Milky Way. For every square meter of Earth’s surface, about 800 million viruses cling to dust and land every day, and we know almost nothing about most of them.
The virus leaves the host and is completely inert. However, viruses can survive outside of their host for longer or shorter periods of time. The short one is only a few seconds, the long one can live for decades. An important factor affecting its survival time is temperature, in addition to sunlight, pH, etc. In very hot environments, viruses tend to die quickly, which is why heat kills them. It could even explain why our bodies evolved fevers as a response to viral infections. In cooler temperatures, the virus can survive for months or even years away from the host. Smallpox virus can survive for decades at temperatures of 4°C to 5°C.
Once inside the host, the virus goes into action. It hijacks the host’s cells and replicates its own genetic material. The host of a virus can be any kind of organism, animal, plant, bacteria, or even another virus. For example, a small virus called “Sputnik virus” inhabits a giant virus called “Mama virus.” Viruses replicate very rapidly, starting within hours of infection; within a few days, their tracks have spread throughout all cells of the host.
Although viruses are inactive outside the host, their behavior becomes incredibly complex once inside the host. Some viruses, including those that cause influenza, measles and polio, enter the host and send out signals to coordinate the actions of their cohorts. For example, deciding whether to attack or temporarily dormant. Different kinds of viruses have their own secret signals. In addition, they can sense communication signals between host cells or other viruses.
We can use this discovery to fight disease. In fact, researchers have engineered viral spies to sense signals specific to other microbes, including E. coli and Salmonella, and then target and destroy them. We could one day use a similar approach to manipulate bacteriophages, a class of viruses that specialize in infecting bacteria, to kill extremely difficult drug-resistant bacteria.
Mapping the Viral Circle
Despite our preconceived notions that viruses can only do harm to their hosts, in fact, viruses can also bring benefits to some hosts. Take, for example, a fungus called Pseudomonas destructive. When the fungus is infected with a virus, it produces more spores, so the virus instead helps it spread.
The interaction between virus and host is the research field of virus ecology. Given the intricacies of their relationship, that’s a lot of work, but we’ve made some progress. In 2017, viral ecologists produced the first map of virus-host interactions, covering all virus species known at the time. Studies have shown that most viruses “choose” to be very specific, and they tend to infect only one or two hosts. For example, a virus that can infect dogs may not do anything to cats unless it mutates. Another advance in viral ecology in recent years has been the expansion of the study of the virosphere into a more mysterious realm—a world of viruses that specialize in infecting archaea.
However, this is just the tip of the iceberg. We have long suspected that viruses are the most diverse microbes on earth, but we have only a vague idea of how many. In the past 20 years, more types of viruses have been identified than ever before. For example, we know about the existence of giant viruses. The virus has more than 1,000 genes, compared with 10 for the smallest virus. As of April 2019, researchers had identified 195,000 viruses.
But there are still plenty of viruses out of our grasp. As analytical techniques have improved, researchers can now identify the genes of the virus without having to find the entire virus. When using this method, they often find large numbers of genetic fragments of unknown viruses.
When the host of the virus is humans themselves, studying virus ecology can help us discover potential threats in time and predict which viruses may jump to the host to infect us (see Extended reading: Which viruses might the next outbreak come from?).
Viruses have also played a major role
Our impressions about viruses are almost all negative, but in fact, viruses have played a major role in the evolution of life on earth.
Viruses are evolving much faster than any other organism, at least a million times faster than us. Evolutionary biologists are beginning to realize that it is the constant stream of new genetic material provided by viruses that has helped other organisms such as animals, plants and bacteria evolve.
Viruses share their genes with their hosts through a process called “horizontal gene transfer.” You can think of horizontal gene transfer as a card swap in poker, where players can optimize their decks by swapping the cards in their hands. If one of the two teaming players happens to have a card that can be constantly refurbished (virus), then they can build a more competitive alliance by switching cards.
Scientists speculate that it was the rapid evolution of viruses, coupled with the ability to swap genes, that allowed early single-celled organisms on Earth to quickly adapt to almost any environment. This was critical for the earliest life, given the harsh conditions of early Earth, and viruses may have played a major role in their success.
Therefore, learning more about the relationship between viruses, hosts and their environments can give us a better understanding of the evolution and even the origin of life.
What viruses might the next outbreak come from?
It is estimated that there are as many as 800,000 viruses in animals that can infect humans. While identifying them is an extremely complex task, we still have a rough idea of which viruses to focus on.
In 2018, the Johns Hopkins Center for Health Security in the United States published an interview with more than 120 infectious disease experts around the world. They agreed that the global outbreak was most likely caused by a new virus transmitted from animals. “New” means we are not immune to it. They also believe that it spreads from person to person through coughing and sneezing, and that it may also spread through asymptomatic infected people.
This all sounds familiar, doesn’t it? Yes, this new crown epidemic completely confirmed their predictions.
Viruses can be divided into DNA viruses and RNA viruses according to the genetic material in their bodies. Experts believe that RNA viruses (coronaviruses are RNA viruses) are more dangerous than DNA viruses. Because DNA viruses are more stable, they are generally less likely to mutate into lethal strains—with one exception, smallpox, the DNA virus that has plagued human history for more than 2,000 years.
Of the 168 known virus families, experts believe that we should pay close attention to five of them, which could become the protagonists of the next outbreak. One of them is the coronavirus (their prediction is confirmed again). The other 4 are:
Picornavirus
The most common of the RNA viruses is the rhinovirus, which usually causes the common cold, and the other enterovirus, which causes a range of illnesses, including colds, polio and hand, foot and mouth disease.
Because they usually cause only mild symptoms, the dangers of this family of viruses have been largely ignored. But they have the potential to mutate into more dangerous strains. The coronavirus is the best example of this. Most coronaviruses only cause colds, but newly mutated strains have caused deadly infectious diseases like SARS, MERS and covid-19.
Currently, little is known about the ability of picornaviruses to jump between species. This outbreak has confirmed that the new coronavirus can jump back and forth on humans and bats, cats, dogs, minks and other animals. There is evidence that picornaviruses can also jump from cows and gorillas to humans.
pneumovirus
Pneumoviruses include human metapneumoviruses, which are transmitted by droplets and usually cause colds but can also cause bronchitis and pneumonia. Human metapneumoviruses are thought to have originated in birds, but research on pneumoviruses has only just begun, and its transmission routes are still being explored. But scientists predict that more deadly pneumoviruses may spread among humans in the future.
Paramyxovirus
Measles, mumps and croup (a childhood disease characterized by a severe cough and difficulty breathing) are all caused by paramyxoviruses. The measles virus is said to have been transmitted from cattle to humans more than 800 years ago. There are also examples of paramyxoviruses jumping from bats, horses and pigs to humans.
Orthomyxovirus
Influenza viruses fall into this category. Influenza viruses have caused several worldwide pandemics. In the 1918-1919 world pandemic alone, the death toll reached 20 million, which was extremely harmful to human life and health. The flu virus has all the killer traits you don’t want it to have, like it’s contagious before symptoms appear, and it’s easy to mutate and jump between species like birds, pigs, and humans.
Influenza viruses are spread by droplets. People with influenza usually recover within a few days, but young children, the elderly, and the infirm are prone to secondary bacterial infection and have a high mortality rate.