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| A Cold Virus. The structure is similar to that of a polio virus and also foot and mouth. Image created by merging and editing a depth map and a color coded map of Rhinovirus B14 capsid proteins by Jean-Yves Sgro |
You see, normally when you catch disease, your body’s natural defences kick into action and, if you survive, that defence includes an alarm system. These are antibodies. They remember what infected you in the first place. We have even harnessed this effect as vaccination, bringing serious killers under control.
Our bodies do the same thing whenever we catch a cold. Yet we can catch colds several times a year, for the rest of our lives. Why? I asked myself through gritted teeth and streaming eyes.
What is giving us these colds?
Up to 80% of calls are caused by rhinoviruses. They are very small, even for viruses, with the typical diameter of 30 nm. You would have to string 1 million of them end to end to make a 3 mm long chain. There are at least 100 different types.
A single cold virus has a shell with 20 triangular faces, a shape known as an icosahedron. The shell, technically known as the “capsid”, protects the important instructions inside. These tell a cell how to make more viruses.
As few as 30 individual viruses breathed up your nose from someone else’s sneeze can cause infection! The virus binds to the inner lining in your nose and throat. It creates a pore and slips into a cell. The slightly acid environment inside your cells breaks open the virus. Its instructions are released and convert your cell into a virus factory.
Eventually, the cell bursts and the released viruses go on to infect more of your nose and throat. The irritation makes you sneeze, spreading the virus to others.
This carries on until your body’s natural defences battle and win against the virus infection inside you. Theoretically, you are now protected by antibodies against that virus infection in the future.
Yet you can still catch another cold. So we return to the question, why?
For you and me, our instructions are written in double strands of DNA. Two metres of it in every single one of our cells. Over 30,000 instructions written in 3.2 billion DNA units called base pairs. When our DNA is copied, it is checked and corrected to minimise mistakes. The error rate is as low as one error in 100 million base pairs.
A rhinovirus’ instruction is only 7000 or so bases long. It is made from a single strand of RNA, not DNA. When the cold virus RNA is copied, the error rate is as high as one in 3000. There is a real chance that each copy contains at least one or two changes.
So, returning to that initial moment of infection last Friday. When I breathed in those 30 or so virus particles, each one was slightly different from the other. Some may have been similar to a cold I’d had before. My body’s defences identified those and killed them off quickly.
But there were others sufficiently different. My body didn’t recognise these new invaders until I’d been infected and I’d had a full scale cold.
When you catch a cold, you are not just catching one cold, you are catching several different viruses at the same time. By the time your colds make you sneeze, you are passing on a different mix of viruses to your unfortunate neighbour.
When you have a cold, you are part of evolution in action.
Rhinovirus: http://en.wikipedia.org/wiki/Rhinovirus#Pathogenesis
19-3 Rhinoviruses are single-strand RNA viruses that replicate in the cytoplasm of the host cell: http://www.microbiologytext.com/index.php?module=Book&func=displayarticle&art_id=477
Rhinovirus Genome Evolution during Experimental Human Infection: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0010588
Human Rhinovirus 14 images: http://www.virology.wisc.edu/virusworld/viruslist.php?virus=r14
This carries on until your body’s natural defences battle and win against the virus infection inside you. Theoretically, you are now protected by antibodies against that virus infection in the future.
Yet you can still catch another cold. So we return to the question, why?
It’s because the viruses change. It’s evolution in action. Here. Now.
For you and me, our instructions are written in double strands of DNA. Two metres of it in every single one of our cells. Over 30,000 instructions written in 3.2 billion DNA units called base pairs. When our DNA is copied, it is checked and corrected to minimise mistakes. The error rate is as low as one error in 100 million base pairs.
A rhinovirus’ instruction is only 7000 or so bases long. It is made from a single strand of RNA, not DNA. When the cold virus RNA is copied, the error rate is as high as one in 3000. There is a real chance that each copy contains at least one or two changes.
So, returning to that initial moment of infection last Friday. When I breathed in those 30 or so virus particles, each one was slightly different from the other. Some may have been similar to a cold I’d had before. My body’s defences identified those and killed them off quickly.
But there were others sufficiently different. My body didn’t recognise these new invaders until I’d been infected and I’d had a full scale cold.
When you catch a cold, you are not just catching one cold, you are catching several different viruses at the same time. By the time your colds make you sneeze, you are passing on a different mix of viruses to your unfortunate neighbour.
When you have a cold, you are part of evolution in action.
Further reading:
Rhinovirus: http://en.wikipedia.org/wiki/Rhinovirus#Pathogenesis
19-3 Rhinoviruses are single-strand RNA viruses that replicate in the cytoplasm of the host cell: http://www.microbiologytext.com/index.php?module=Book&func=displayarticle&art_id=477
Rhinovirus Genome Evolution during Experimental Human Infection: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0010588
Human Rhinovirus 14 images: http://www.virology.wisc.edu/virusworld/viruslist.php?virus=r14
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