Are viruses alive? And other virus-related questions
From colds to cold sores, chickenpox and COVID-19, when we think about well-known infections or diseases, there's often a virus behind them.
In fact, viruses are the most diverse and numerous disease-causing organisms on Earth, infecting animals, plants, fungi and bacteria - as well as humans. Scientists have estimated that if each individual virus on Earth were lined up side-by-side, they could easily stretch from one end of our galaxy to the other. When you consider that viruses range in size from around 20 to 800 nanometres, there must be a lot of them!
And yet, for something so widespread on our planet, there's actually quite a lot we don't often hear about viruses. With the coronavirus pandemic they've become a hot topic in the news, so we've asked Dr Bronwen Burton, lecturer in immunology at the University of Bristol, to answer some questions about viruses for us.
How and when did we discover viruses?
The invention of the microscope allowed us to start looking at very small things known as microbes or microorganisms. These include bacteria and fungi, but individual viruses are too small to see with an ordinary microscope. Bacteria are roughly 20-times smaller than human cells, while viruses are around 100-times smaller.
This size difference helped scientists in the late 19th Century begin the slow process of discovering viruses. They noticed that filters used to remove bacteria from liquids couldn’t get rid of all disease-causing microbes, because some were too small to be trapped.
Later, the structure of these mysterious microbes was discovered. Viruses have a very simple form; a DNA or RNA genome - genetic code - surrounded by a protective protein coat. Some viruses, like coronaviruses, have an extra fatty envelope to keep their genetic material safe. This is vital because like all living creatures, viruses aim to pass on their genes by reproducing.
Are viruses alive?
The usual answer to this question (and usually for the purpose of passing your Biology GCSEs) is that viruses are not alive, because they do not complete all of the seven life processes: Movement, Respiration, Sensitivity, Nutrition, Excretion, Reproduction and Growth.
However, viruses have genetic information coded in DNA or RNA, a characteristic shared by every other living thing. So does that mean they can be considered ‘alive’? Let’s take a look at what biologists mean by ‘alive’.
We can agree that dogs, for example, are living creatures; they grow, reproduce, release energy from nutrients, move and respond to the world around them. They also excrete waste products (including poo). But viruses don’t show all these characteristics.
Viruses can’t move, grow, convert nutrients into energy or excrete waste products. But viruses certainly reproduce, infecting people and causing illnesses. It’s how they reproduce that’s unusual.
Viruses lack essential machinery needed to reproduce by themselves. In fact, viruses can only reproduce after infecting a living cell - a process called viral replication. Once inside a living cell, viruses re-program the cell’s machinery to produce viral proteins and genetic material to make new copies of themselves. Viruses with an envelope steal a fatty layer from the cell. Then, new virus particles infect other cells, turning them into virus production factories too.
So viruses are unlike any living creature in how they reproduce. Even single cell organisms like bacteria can reproduce independently on surfaces outside the body, but viruses can only survive for a while outside host cells. A person infected with a cold virus might sneeze, transferring virus onto surfaces around them, like a doorknob. The longer the virus is on the doorknob the less able it will be to cause a new infection.
Another characteristic of life is the ability of a living population to evolve over time, gaining changes to genetic material. Dogs evolved from a wolf ancestor, developing from savage hunters to faithful pets. But what about viruses?
Where did viruses come from?
Viruses have been around for a few billion years, but it’s not clear which evolved first: viruses or cells. Three theories try to explain where viruses came from. The first suggests that genes encoding viruses might originally have come from cells, like bacteria. Small sections of DNA may have escaped from a cell’s genome, eventually gaining a protein coat: bingo! The first virus.
The second theory suggests viruses evolved from an ancient single-cell organism that stopped being able to reproduce by itself, becoming dependent on host cells instead.
The final hypothesis proposes that viruses existed before cells; ancient viruses may have evolved over time to produce membranes and cell walls, giving rise to living cells. It’s possible that all these theories are right.
What would happen if all viruses disappeared?
Viruses have led to pandemics and death, which has caused them to have, it's fair to say, a pretty bad reputation. But not all viruses are harmful. Millions of years of evolution alongside host cells mean that viruses are very selective about which type of cells they infect (for example, plant viruses don’t cause disease in humans), and some viruses may provide benefits to their hosts. In fact, some scientists think that if all viruses magically disappeared one day, it would end life as we know it.
Viruses help maintain ecosystems by making sure that individual populations (such as insects) don’t outcompete other populations, causing a huge crash in biodiversity. Viruses kill around 20% of microbes in the oceans daily, releasing nutrients from dead cells which help feed other microbes. This is essential because microbes in the oceans produce about half of the oxygen on Earth. Viruses also affect carbon dioxide levels on Earth by helping to hide carbon deep in the oceans.
You’ve probably heard of ‘good bacteria’ which help keep your guts healthy. Viruses may help to regulate bacteria in your guts and train the immune system to fight some infections. Viruses can be used as tools in medicine, for example bacteriophages - viruses which infect bacteria. There should, in theory, be a bacteriophage capable of killing every single bacterium on Earth. If we can match the right bacteriophage to an antibiotic-resistant bacterium, we could treat untreatable-infections. Viruses can be used to deliver correct copies of defective genes into cells during gene therapy and research even suggests that viruses that specifically kill tumour cells might help us to fight cancer.
We’ve probably only discovered a tiny fraction of the viruses on our planet, with most research focusing on disease. But the more we learn about viruses, the more we understand their importance in the life of the planet.