Intro to

An Introduction to Viruses

A back to basics explanation of what viruses are, how they infect and how they spread.

Virus is the word of the day at the moment and there’s lots of misinformation floating around about COVID-19 so I thought I’d give an introduction to viruses. This is by no means everything there is to know but it will hopefully give you some background into what viruses are and what they do.

If any questions arise while you’re reading, feel free to drop them in the comments below.   

Viruses need a host

There is an ongoing debate on whether viruses are alive or not. I could write an entire blog post about this argument alone and definitely will.

Cells constantly copy themselves (replicate). Your cells do it, my cells do it. Viruses do it too.

However, unlike our cells, viruses can’t do it alone.

Viruses can’t survive by themselves, they need what’s called a host. Viruses are parasites, they hijack the cells of living things and use them to produce energy and to carry out the processes needed to replicate. 

But let’s talk about how a tiny virus can hijack our cells and the cells of a whole range of living things. 

What are they made of?

The entire purpose of a virus is to replicate and infect more cells. That is everything they exist to do. They want to spread their genetic information far and wide. 

Viruses store their genetic information in two different ways depending on the virus. One way is as DNA (that world famous molecule), the other is as RNA, a very similar  molecule to DNA. These molecules are called nucleic acids. 

I’m going to talk about genetic material for the rest of the post, by genetic material I mean the nucleic acid, which could be DNA or RNA, this is so that I can talk about all viruses.

 Our cells are able to make both DNA and RNA so viruses are able to use our machinery to copy its genetic material. A virus’ genetic material acts as an instruction manual on how to build a virus. Every virus particle carries round this instruction manual.

However, genetic material is incredibly delicate and can be damaged easily. Without the instruction manual, viruses can’t do the one thing they exist to do: replicate. So it makes sense that it wants to protect it. Viruses use something called a capsid to protect its genetic material. 

Some viruses add an extra layer of protection, called an envelope. Viruses that don’t have an envelope are either called non-enveloped (creative, I know) or naked viruses. Some viruses use an envelope to help them get into cells. Both enveloped and non-enveloped viruses can also have spike proteins (more on these later). 

A diagram of a naked and enveloped virus. Both have a capsid filled with genetic material. The enveloped virus (right) has a coating surrounding the capsid with spike proteins attached around the outside.
Viruses can be naked (left) or enveloped (right). All viruses have a capsid that protects the genetic material. Some viruses also have spike proteins, these can be found on naked or enveloped viruses.

Getting into your body 

So a virus has its genetic material protected, now it needs to get inside a host because we know it can’t survive for long out in the open. 

Viruses can enter the human body a number of ways, the most common being through the respiratory system, the stomach or through the exchange of bodily fluids. 

Once inside the body, viruses find their way to their target host cell. 

How do they infect? 

But how does a virus know it’s found the target and why can’t it just infect any cell? 

Well viruses have things on their surface called spike proteins. These act as biological keys. On every cell in your body you have what’s called a cell membrane surrounding it. This cell membrane acts as the gate-keeper to your cells, making sure nothing gets in that shouldn’t. It’s like a locked door. Only molecules that have a matching key can find their way into the cell. 

Viruses have evolved to possess that key. When the key (spike protein) fits into the lock (receptor) on the cell membrane, it enters the cell. Enveloped viruses add to this because the envelope has the same makeup as the cell membrane, allowing it to fuse with membrane, releasing the capsid containing the viral genetic material into the cell. 

Spike proteins can be found on naked and enveloped viruses.

A diagram demonstrating the binding of spike proteins to receptors. On the left shows a virus spike protein matching a receptor on a human cell. On the right shows the same spike protein that doesn't match a differently shaped receptor.
Viruses use spike proteins to bind the receptors on cells. If the shape of the spike protein matches the shape of the receptor (like a key in a lock), then the virus will be able to enter the cell (left). If the spike protein doesn’t match the receptor, the virus won’t be able to enter the cell (right). This is why viruses only infect specific cells.

Viruses only contain the key to unlock specific cells, this is why viruses have target cells. For SARS-CoV-2 (this is the name of the virus that causes the disease COVID-19), its spike protein matches the ACE2 receptor. This receptor is found on cells in the lung, kidney and gut. ACE2 normally binds something called angiotensin-II and is important for regulating blood pressure but SARS-CoV-2 is able to trick the cell into thinking that it is angiotensin-II and the cell allows it to enter. 

What they do once they’ve infected a cell.

So, once a virus has found its way into the host cell it needs to start making copies. 

Viruses don’t have the machinery needed to do this but lucky for them the host cell has the machinery to do everything they need! Once in the cell, the viral capsid breaks apart releasing the genetic material. The virus then uses the host cell machinery to copy its genetic material and make the all important capsid. 

After the new virus particles have been made, they can exit the cell in two ways depending on the type of virus.

One way is that the virus makes so many copies of itself that the cell bursts, this is called lysis. This is how viruses can make you ill because they damage and kill cells. 

Another way is used by enveloped viruses. The capsid pushes its way out of the cell through the cell membrane and takes some of the membrane with it, forming the envelope. This process is called budding

The virus then goes on to infect more cells. 

A diagram illustrating how a virus replicates and exits the cell. Shows genetic material passing through a machine on one side and capsids exiting on the other. These capsids then go on to burst the cell open or push out of the cell.
Once viruses enter our cells, they hijack our machinery and use it to make copies of themselves. They can then exit the cell by lysis (bursting the cell open) or budding (pushing out through the cell membrane).

Some viruses once they’ve infected a cell are pretty sneaky. They go into what’s called a latent phase. In this latent phase the virus lays low and does the bare minimum in the cell, then later on it switches back to using the host machinery to copy itself and infect other cells. Some viruses that do this include HIV and varicella zoster (chicken pox) virus. 

There is so much more I could talk about when it comes to viruses. They are pretty amazing. But I hope this very condensed version has given you some useful information about what viruses are and how they survive in a host. 

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