How can something so tiny be wreaking such havoc? SARS-CoV2 is the name of the virus that causes the new disease Covid-19, named on February 11.

t is so small that 500 million of them would fit on the full stop at the end of this sentence. David and Goliath, except David isn't even as big as an ant when compared to Goliath. And yet look what's happening. Economic turmoil. Cities and towns in quarantine. People not travelling for holidays or weddings. Sporting events cancelled. People who feel sick and who have met someone with the virus keeping themselves in isolation for 14 days. And the fear that the worst is yet to come outside China where it all started. That something so tiny can pack such a punch is a testament to how powerful viruses can be.

Viruses were first observed in 1948 with an especially powerful microscope called the electron microscope. The first members of the viral rogues gallery to be seen were the viruses that cause polio and smallpox. Both are highly contagious (around three-fold more than SARS-CoV2) and wreaked havoc in humans for centuries, paralysing us, disfiguring us and killing us. Then vaccines were developed and that put an end to that, with smallpox being eradicated completely and polio almost beaten.

Apart from being able to see them, scientists also figured out what viruses were made of. They have a coat made of fat, so they don't dissolve in water, although alcohol can dissolve them, which is why alcohol hand rubs are good at killing them.The alcohol dissolves the fat. Inside the fatty bag lies their genetic material - the recipe that can be read to make more virus. They also have proteins sticking out of the bag and they use these proteins to latch on to the cells they want to infect. A bit like a key, the protein fits into a lock on the surface of the cell the virus wants to infect and opens the door.

In the case of SARS-CoV2, the proteins occur on the end of the spikes that make the crown that surrounds them. This is why it's called a corona virus. They stick the spike key into a lock called ACE2 on your lung cells and the virus then gets inside. This is why it infects your lungs: that's where the ACE2 lock is.

It needs to get inside the cell to use it as a factory to make more viruses.

The ultimate parasite

Viruses are the ultimate parasite. As far as we know they bring no benefits. A bit like unwelcome guests who come to stay, procreate in your guest room having eaten all your food and drunk your wine, and then leave without saying thanks.

The recipe that SARS-CoV2 has to make more of itself is called RNA. This is why SARS-CoV2 is a bit like flu - the influenza virus also has RNA as its recipe, as do viruses that cause the common cold and Aids. There are plenty of types of RNA viruses.

Once it's made copies of itself, it leaves and moves on to another cell. The trouble is, it sometimes kills the cell it infected - the guests leave a bomb as they depart - and that's when the trouble can begin. You start to hurt. Influenza will kill billions of cells in your lungs in a typical infection, which causes fluids to build up making it hard to breathe. That can really irritate your lungs. And then you cough it out. The virus makes you cough because it wants to spread. The drops of spit fly through the air and land on surfaces where someone else picks them up and then touches their nose or mouth and the virus enters a new body. The unwanted guests have moved next door.

This is why it's important to wear a mask if you're infected since that traps the virus. And why the number one recommendation of the World Health Organisation is to wash your hands. And why it's good to clean surfaces if you've someone in your house who's infected. Wearing a mask doesn't seem to protect people much as they fidget with it or take it off a lot. And the virus can probably get in through your eyes anyway.

Natural defence

But now some good news. Luckily evolution has helped you. Your immune system is on hand to recognise the intruder and bring out the big guns to kill it. It's like you've got on your iPhone and called for the gardai to get rid of your unwelcome guests (if possible, before they have done the deed in your guest room).

The immune system has evolved all kinds of ways to recognise and eliminate the intruder. It has special sensors for the virus's RNA which set off the alarm. It can also detect the spike protein. Your immune system can make antibodies and these latch on and stop the virus getting into cells. A bit like putting blu-tack over the key. The antibodies also help immune cells eat the virus.

Your immune system even has a way of killing the virally-infected cell. This is almost like the gardai deciding to blow up your house. It is worth it because it stops the virus (or your guests) moving into other houses in your neighbourhood. Remember, they've multiplied. So blowing up one house saves many.

If you're healthy, your immune system works a treat. The gardai are well fed, have had a good night's sleep and have the weapons to do their job. And, once the job is done, they are highly experienced. Should the unwanted virus turn up again, they can recognise and kill it on sight. This is how vaccines work. They are weakened forms of a virus, or parts of it, which train the immune system so that when the real culprit comes along, the immune system is ready to attack and you are protected.

So what can go wrong?

In the case of Covid-19 (and influenza), people who are sick with other ailments (for example cancer or heart disease) can't mount a proper defense and so the virus runs riot. Their immune systems aren't up to the job because of the other illnesses they have. Sadly, this can mean fatalities which at this stage are around 2pc and mainly involve people with other illnesses. As we age, our immune system does, too, so this puts older people at risk. We therefore need a vaccine and huge efforts are going into that with the real hope that one will be available in nine-12 months.

Doctors are also testing medicines to stop the virus from harming us. Drugs used to treat HIV are showing promise; HIV is somewhat similar because it has RNA too.

A drug used to treat malaria called chloroquine is also showing promise, as are high doses of steroids. What these drugs do is interesting. Although the immune system is failing in people who get really sick, it turns out that one part is over-active. Because the virus is running rampant, it hugely provokes this part (called innate immunity) which causes a process called inflammation to kick off - this makes your temperature go really high and causes your lungs and other organs to fail.

What people actually die of is the friendly fire caused by this over-active inflammatory response which is sometimes called a 'Cytokine Storm'. Steroids and chloroquine put that fire out and so protect you.

It's a bit like where there were two unwanted visitors in your house, there are now thousands and the gardai get their batons out and go to work on them. A melee ensues and sadly in the violence and chaos you die. Not a good result. Steroids and chloroquine are like cold water being sprayed over the gardai.

What next?

SARS-CoV2 is a new virus so we have to be vigilant. The death rate is unlikely to go up and if anything might go down as more people are found to have fought it. It also mutates at a rate slower then say HIV or influenza so it can't change itself too readily.

This means that once your immune system recognises it and eliminates it, it will recognise it again. A change might also mean it becomes more toxic, killing more, but again this is unlikely.

It may well enter the community and become just another virus that causes flu-like symptoms that we learn to live with. It might weaken as it adapts to us. Killing us is in general a bad idea for a virus - it's like those guests... why would they kill you when they want to sponge off you again? Many will develop resistance and refuse the unwanted guests entry. And when we have a vaccine, the vulnerable can be protected.

Right now though, follow the guidelines. Isolate yourself if you have symptoms and have come into contact with someone with the virus and call your GP. No need if you don't meet these criteria. Wash your hands a lot.

Soap and water is fine -work up a good lather as viruses hate soap because it dissolves them. If you're vulnerable, don't travel to places where the virus is. We all just need to keep calm, remain vigilant and wait it out. This too will pass.

Luke O'Neill is professor of biochemistry in the School of Biochemistry and Immunology at Trinity College Dublin

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Coronavirus Ireland: What exactly is Covid-19 and what will happen next? Biochemistry expert Prof Luke O'Neill explains - Independent.ie