What is Flu?

How does the flu virus make us ill?
25 September 2018

Interview with 

Wendy Barclay, Imperial College London




What actually is the flu and where did it come from? Chris Smith spoke to Wendy Barclay who is a virologist at Imperial College London…

Wendy - Flu is a disease caused by a virus - the influenza virus. That virus gets into our bodies when we breathe in droplets from somebody else who’s been infected by the virus. And then those droplets containing the virus go down into our nose and throat and then into our deeper lungs perhaps, and like all viruses it’s an absolute parasite so it actually has to get inside our cells. And once the virus is inside our cells it takes over the cells, reproduces itself, kills the cell and then thousands of new viruses, copies of the original come out and spread.

Chris - And how does it actually make us ill? Why do we feel so ghastly when we’ve got it?

Wendy - I think there are two ways. Some of the first symptoms are sore throat. That could well be just because the virus, in replicating in those first few cells in your throat, have killed that important protective barrier which normally protects from incoming dust particles etc. And if you haven’t got those working then all those dust particles which are in the air can really hurt as they land on the nerves below.

But the other reason that you feel so really bad with flu is that your own immune system recognises that these cells have been invaded by a virus and responds by releasing chemical messengers. And they also, at the same time as telling the cells where to come, induce fever and aches and pains, lethargy, feeling hot, because your blood is now full of all these chemicals which have been released from the infected cells and are trying to signal to other immune cells to come and help.

Chris - Now, where did flu come from in the first place?

Wendy - All flu viruses are not human at all, they are bird viruses. There are lots and lots of different types of flu, which means that antibodies against one wouldn’t protect you against another. And they’re all out there, at least 16 of them, sitting in the wild birds  of the world - ducks and geese and also seabirds that migrate through very large distances, but also live in huge colonies. A fantastic place for a virus to hang out because there’s always new hosts for it to infect. It actually is a virus that infects the intestines of those birds and it comes out into the water, the birds all land on the lakes, and the water and the lakes is full of flu viruses that the birds can drink and take up and get re-infected.

Chris - So how did it get from being a bird virus to being a human virus, and when did that happen do you think?

Wendy - We know from historical records that pandemics, probably of flu virus, happened thousands of years ago, probably when humans started living in larger numbers in cities.

How does a virus transform from being a bird flu into a human flu is a matter of intense research, because it will help us ultimately predict the chances of pandemics happening, and it’s certainly not a single step. There are at least three changes and probably more that a bird virus would need to make to be a successful human virus. And it’s a bit like rolling a dice, if you want to get three sixes all at once, that’s not the most likely and therefore we don’t get pandemics all that often, but out there in nature that dice is being rolled all the time.

Chris - Is it fair to summarise then and say we’ve got this flu virus. It started off in birds; at some point it jumped into humans and we ended up with human forms of flu, which we keep on handing on to each other, year on year, but there remains this enormous reservoir of bird viruses that periodically can do that jump again, and when they do that jump again then we get a new kind of flu in humans?

Wendy - Yeah, that’s absolutely it. Certainly what we know is that in 1918 a virus came across that had recent ancestry in birds, became a human virus, and then stayed in humans for the next three or four decades.

Chris - Do we know what factors probably encouraged that jump in 1918 to make that virus seed into humans and produce that devastating pandemic?

Wendy - We don’t know for sure whether or not the circumstances that were present in 1918 were the perfect storm, if you like, for that virus, to make that jump. Certainly there are theories that when so many young men were moved into these huge army camps and lived in very close quarters, the chances of a virus accumulating the right numbers of mutations and then spreading onwards to others were increased. So there is some theory that that special circumstance would have allowed the virus to emerge in that way.

Chris - A number of people have looked at a cross section of the population who succumbed to the 1918 flu and it looked a bit unusual because in most winters you get lots of young people and lots of elderly vulnerable people who will succumb to flu, but with this we saw lots of previously hale and hearty young people dying of this. Why would there be that difference?

Wendy - Yeah, it’s a really excellent question. I think there are two main theories. The first is the cytokine storm theory. So that says that much of the symptoms of flu in a human is down to the person’s own immune system, as we’ve discussed, of sending out those chemical signals and responding to the viral infection. And in 1918 flu infections, those responses may well have been inappropriately big. If that’s the case, the people who are the healthiest and would make the biggest immune response are the people who would get very sick. So 25 to 45 year olds with their healthy immune system, kind of overreacted and consequently ended up that the lungs were full of inflammatory cells.

The other theory relies very much on this idea which is quite popular at the moment, of the flu that you experience in your very early life kind of sets the scene for the rest of your life and affects the way you respond to subsequent flus. So if we historically trace back to when previous pandemics were recorded in the world. Certainly in 1889 we think there was a previous flu pandemic and that may have somehow set the scene for people to respond differently to the next pandemic virus that came along.


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