The recent emergence of new coronaviruses is responsible for the COVID-19 pandemic, which draws attention to the risks that animals as a new virus source may pose to humans. The virus is called SARS-CoV-2, and it is currently not possible to determine the origin of the human version of the virus. Bats are considered to be the most similar animals to known viruses, but it is also uncertain that bats are a direct source of SARS-CoV-2.
So, how did new viruses emerge from the environment and begin to infect humans? Each virus has its own unique origin in time and mechanism, but there are some common facts about all new viruses.
The first thing to know is that it is rare for viruses to jump between species. In order for the virus to successfully jump to a new host species, it must be able to do several things.
First, it must be able to establish infection by replicating itself in the new host. This is not an established process because many viruses can only infect certain types of cells, such as lung cells or kidney cells. When a virus attacks a cell, the virus binds to specific receptor molecules on the cell surface, so it may not be able to bind to other types of cells. Or the virus may not be able to replicate in the cell for some reason.
Once infected with a new host, the virus must be able to replicate itself enough to infect others and spread itself to others. This is also very rare, most virus jumps will lead to what we call "end hosts", where viruses cannot spread themselves and eventually die.
For example, the influenza virus H5N1, or "avian influenza," can infect humans from birds, but transmission between humans is very limited. Sometimes, this obstacle is overcome, and emerging viruses can jump to new hosts, establish new chains of transmission and new outbreaks.
From research in the past few decades, we have learned some of the mechanisms that cause viruses to jump between species. The flu virus is a typical example. This virus contains eight genomic fragments. If two different viruses infect the same cell, the fragments of the two viruses will mix to form a new type of virus. If the protein on the surface of the new virus has changed significantly compared with the current influenza virus strain, then no one has immunity and the new virus can easily spread.
This transfer of influenza virus is called antigen transfer. This is what we think happened in the H1N1 flu epidemic in 2009, the transformation that occurred in pigs, and then jumped on people and started to break out. There is also genetic evidence that this mechanism may occur in coronaviruses, although its role in the emergence of SARS-CoV-2 remains to be determined.
New viruses can also arise through genetic mutations in the viral genome, which is more common in viruses because viruses store their genetic information in similar molecular RNA, not DNA. This is because these viruses (except coronaviruses) lack a method of checking for errors when replicating. Most mutations generated during replication will cause damage to the virus, but some mutations will make the virus more effectively infect new hosts.
So what do we think happened in the SARS-CoV-2 case? Recent analysis of the genome has shown that the virus has spread in a very similar form for about 40 years. The closest relatives of the virus we can recognize were found in bats. However, this virus and SARS-CoV-2 may have a common ancestor about 40-70 years ago, so this bat virus is not the cause of this outbreak.
Although these viruses have a common ancestor, the evolution over the past 40 years has separated them. This means that SARS-CoV-2 may be transmitted to humans by bats, or may be transmitted to humans through intermediate species. For example, viruses closely related to pangolins have been found. However, until we can find more closely related species in the environment, the exact path of genetically distinct SARS-CoV-2 will remain a mystery.
It is unclear what changed the virus so that it can infect humans so easily. However, given that in the past 20 years, the Coronavirus family has seen three major diseases-SARS, MERS, and COVID-19, it is likely that this is not the last time the coronavirus infects humans and triggers new outbreaks.
It is more likely that when the virus spreads in all animals in the world, the virus will only spread to humans if they have the opportunity to infect humans through contact between us and other animals. In the process of exploring new areas and spreading to the world, human beings will always be exposed to new viruses. But the increase in human activities in the wild and the trade in wild animals have created a perfect breeding ground.
Our global connections enable a new disease to spread throughout the world in a few days, which also complicates the situation. We must bear some responsibility for these emergencies because we continue to interfere with the natural environment and increase the likelihood of viruses infecting humans.