Viruses are in all living

VIRUSES

– a part of all life?

Viruses, are they a type of life? No, explains Peter Norberg, Associate Professor at the Institute of Biomedicine. – Viruses comprise their very own and incredibly interesting group. They are usually relatively harmless but can sometimes cause serious illness. They are also an important reason why we humans, on the whole, exist as we do today.

Text: Eva Lundgren Photo: Johan Wingborg

– Today there are approximately 7.7 billion people on the planet, who can be considered to be a single population. This means that the conditions for new pandemics are very good, explains virus researcher Peter Norberg.

Anyone who studies the tree of life will search in vain for the branch from which viruses developed. And there are several reasons why they are not included among living organisms, explains virus researcher Peter Norberg. – Among other things, they cannot live independently, but require a host cell to replicate. All viruses have some form of surface structure that specializes in binding to a particular cell type. Once inside the cell, the virus reprogrammes the cell so that it starts to produce more virus particles. Viruses are also very small, somewhere between 10 and 300 nanometres, and have a genome consisting of a few thousand to a million bases. Some viruses have an envelope that makes them susceptible to alcohol, which is why we are advised to spray our hands to prevent the spread of COVID-19. Others do not have an envelope and can therefore not be inactivated with alcohol, for example the norovirus that causes winter vomiting disease. However, these can be washed away mechanically with soap and water.

There are three theories about where viruses come from, says Peter Norberg. – The progressive hypothesis proposes that viruses are genetic material on the loose, which has somehow escaped from various organisms. The regressive hypothesis instead proposes that viruses were previously more complicated and independent, but that they underwent a simplification. Another hypothesis is that viruses are more primitive than living organisms, and that in some kind of primordial stage they survived on their own, without host cells. Other theories consist of combinations of these three hypotheses.

All types of life that have been studied – animals, plants, fungi and bacteria – have their own types of viruses. Virus particles are probably the type of organism that is by far the most abundant on earth.

Viruses can be transmitted in a variety of ways, for example through air or aerosol spread, contact infection, via blood, secretions or organ donation, to mention but a few. In order for the spread within a group to work over a longer period of time, a fairly large population is required, Peter Norberg explains. – In a small population, as in the prehistoric hunter-gatherer groups, the virus would have spread rapidly to most individuals, which would have led to the entire population developing antibodies and the virus dying out, so-called herd immunity. However, it can survive elsewhere, for example in bats, and then after a number of years re-infect the new members of a community, who do not have antibodies.

Some viruses can also remain latent in an organism. For example, chickenpox lies dormant in the ganglia of all infected people for the rest of their lives. Eventually, the virus can flare up again in the form of shingles, and then pass the infection on to children who lack immunity.

The development of agriculture led to a huge increase in the population, and thus new opportunities for viruses to spread.

The virus transmitted the genes that provide the ability to build up the membrane in the placenta that separates the mother and foetus, which allows an embryo to develop inside the body.

PETER NORBERG

– Several of the viral diseases that today exclusively affect humans, such as many cold viruses, measles, HIV and COVID-19, have zoonotic origins, which means the infection has come from animals. As there are currently approximately 7.7 billion people on the planet, and it is so easy to travel across the globe, humanity can be seen as a single population. This means that the conditions for new pandemics are very good.

The fact that many wild animals, which would never have encountered one another in the wild, are caught, forced to live in crammed and poor conditions in cages and then sold increases the risks even further, of course. The felling of forests and other types of encroachment into nature also means that certain animal species are forced to move, and can thus carry viruses to new places.

Two thirds of the first 40 people who were reported to have been infected with the SARS-CoV-2 virus had visited a specific market in the Chinese province of Wuhan, says Peter Norberg. – This has been interpreted as meaning that the spread of infection began in this market. On the other hand, a third seem to have been infected in some other way, which could be interpreted as the disease actually having a different origin.

For a long time, researchers believed that the infection came from bats but quite early on, the researchers discovered another probable vector: the pangolin, or scaly anteater . – An important part of SARS-CoV-2, namely the receptor-binding part of the spike protein itself, is almost identical to a coronavirus in pangolins. One explanation for the origin of COVID-19 could be so-called recombination, i.e. that two closely related viruses infected the same cell in the same host, which caused the genes to mix and form a new virus. However, the closely related coronaviruses found in bats and pangolins are not completely identical to SARS-CoV-2, which means that we probably missed a small step in the development of this virus. It has also been discovered that minks and cats are easily infected by SARS-CoV-2, which has led to speculation that several different animals may have contributed to its emergence.

Viral diseases have caused a great deal of misery throughout human history.

But is there anything good about viruses? – Of course, one example is bacteriophages, i.e. viruses that attack bacteria. In the Soviet Union, bacteriophages were used for a long time instead of penicillin, and given the development of the resistance that we see in bacteria today, it may be relevant again. There are documented cases where bacteriophages have succeeded in eliminating very serious and life-threatening infections caused by multi-resistant bacteria, where antibiotics have not worked.

The virus’s ability to penetrate living cells is also used in gene therapy. – In viral gene therapy, a functioning gene is spliced into a specially designed virus. This virus then produces the protein that the gene encodes in each target cell.

But viruses also seem to be the cause of one of the MAYor changes in life on our planet, Peter Norberg explains. – A retroviruses is a special kind of virus that converts its RNA into DNA. The DNA copy is then inserted into the host cell’s own genome. If the virus happens to be incorporated into egg or sperm cells, so-called endogenous retroviruses, the virus will be inherited by the children of the host cells as well as their grandchildren’s children. This has happened on several occasions during the evolution of life; the fact is that a large part of the human genome comes from these endogenous retroviruses.

When a retrovirus infected an egg-laying vertebrate several hundred million years ago, the consequences were incredibly special. – The virus transmitted the genes that provide the ability to build up the membrane in the placenta that separates the mother and foetus, which allows an embryo to develop inside the body. Therefore mammals, including us humans, have viruses to thank for the fact that we exist at all.