Science Questions

Can you have more than one cold at once?

Tue, 23rd Feb 2016

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Question

Staffan Lincoln asked:

Can you have more than one cold virus at once? How could you tell?

Answer

Kat asked Chris for an answer to this sniffly question...sneeze

Chris - Originally, we thought the answer to that question was "no".  And the reason we thought the answer to that question was no is that when you are infected with a cold, which is a virus, the virus goes into the cells lining your nose and throat (and possibly your lungs) and it triggers what we call an interferon response.  The virus trips over various molecular tripwires inside your cells, triggering the cells to sound an alarm, and that alarm signal spreads to all the other surrounding cells and goes systemically through your body. It puts your other cells into an anti-virus state, making them very hard to infect and very hard, or much harder, for viruses to grow in those cells. So we went around saying, "well, we therefore think itís unlikely that youíll suffer from more than one virus infection at a time."

Now, in the last ten years or so though, certainly in our hospital and almost universally in most first world hospitals, weíve switched over to what we call molecular diagnostics. This is where people send in blood samples, or they send in samples of swabs from their nose and throat when theyíve got various respiratory illnesses, and we extract the DNA or RNA - its chemical relative - from those swabs and then we read the genetic message to see which virus signatures we can pick up.  And what weíre routinely seeing now is that there are people with three, four, in some cases five different virus infections recoverable at the same time.  So, while itís probably true that you will get this very powerful immune response, including the release of this chemical interferon when you have a virus infection, itís also true that you are not put into a completely resistant state and you can succumb to multiple viruses at once.  We know what they are, and we also know that they can do what we call "synergise": if youíve got two things going on at once, they can make you worse than if you just had one thing badly.

Kat - Oh, because Iíve had a really horrific stomach bug over the past sort week or so.  You know the full like up and down and feeling grotty.  I thought, "am I immune to getting something like that again?"

Chris - Well, there are a number of aspects to this. When you have had an infection, then you will develop a neutralising antibody response to the thing you have just succumbed to.  And what that means is your immune system has seen what it looks like, it makes antibodies and it makes an immune memory, which includes making more antibodies for the long term but also white blood cells capable of attacking cells that have got that virus in. So, great, youíre not going to catch that virus again. But, a lot of these virus use a chemical called RNA as their genetic material instead of DNA. And RNA, unlike DNA which is double stranded - one strand is the genetic mirror image of the other and itís like a backup copy, so if something goes wrong with one strand youíve got the backup image to fall back on - RNA is just a single strand of genetic information and if that changes, through a process called mutation, then you change what the virus looks like. Itís like a molecular facelift for a virusÖ And what that means is that you just donít recognise it for what it is and it can come again. This is exactly what the flu does and it can come again, and again, and again. Norovirus, which if youíve had a really bad dose of upset stomach, is a good contender, changes probably faster than any other virus that we know of. Roughly 1 in every 100 genetic letters that it makes is going to have a genetic spelling error in them. And the harsh reality of Norovirus is that in every millilitre of what leaves your body - upwards or downwards - when you're infected, thereís enough virus in that 1 ml to infect the entire population of the world.

Kat - Well thatís a joyful though for us all.

Chris - So itís hard not to catch it!

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Yes. There are hundreds of different viruses that produce the generic "common cold" symptoms; identifying which one you have requires a genetic or antibody test. You could easily catch two viruses at once, just as easily as catching them a week apart.

But there is a particular scenario that is important for influenza epidemics: If humans and animals live in close proximity (eg pigs or ducks on a family farm), it is possible for an animal to catch (say) pig influenza and human influenza at the same time.

Because of the way influenza consists of separate strands of genetic material, it is possible for a hybrid virus to form, containing both pig and human virus. Humans may have little or no resistance to this new virus, so it could cause an epidemic.

Swine flu (pigs), Avian flu (chickens or ducks) and MERs (camels) may well have formed in this way. evan_au, Tue, 19th Jan 2016

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