A battle of epic proportions is raging in your body when you have flu. But precisely what are the forces involved? Is a cold any different? And is there any science behind man flu?
We’re fast approaching the start of flu season – it officially begins in June – and Australia could be set for a big year.
A case of summer flu has seen record infection numbers in the first few months of 2019, with more than 40,000 lab-confirmed cases in Australia. In Victoria, there have been 23 influenza outbreaks in aged-care homes; 17 elderly Victorians and at least one child have already died. NSW had seven flu-related deaths between January and March.
Record quantities of the flu shot have been ordered but doctors are already raising concerns about another possible flu-vaccine shortage.
When flu strikes the symptoms can be awful. But what is actually going on in your body? How does the virus operate? And what is the difference between it and a cold?
What is flu?
The flu – influenza – is a virus. It’s a piece of genetic information, similar to DNA, surrounded by a number of proteins that help it with infection. When it infects us, the virus delivers its genetic information into our cells so that it can hijack them. Our cells are forced to mass produce hundreds of thousands of new copies of the virus that are then released to take over more cells.
There are four similar strains of the influenza virus: A, B, C and D. Thankfully, influenza viruses B and C are uncommon and don’t cause pandemics, and D is not known to infect humans at all. So the A strain is the one we really need to worry about.
"The big problem with influenza A is that it loves to change itself."
The influenza virus has two key tools: think of them as scissors and glue. First, it uses a glue-like protein called haemagglutinin to stick to our cells, and then injects its genetic material inside.
The genetic material hijacks the cell, forcing it to make new copies of the virus. The virus then uses another protein, neuraminidase, to snip those new viruses out so they can go and infect other cells.
The big problem with influenza A is that it loves to change itself. Our immune system recognises viruses by the shape of their proteins. If those proteins change shape, the immune system can't stop them and they can infect us.
The ability of Influenza A to infect other animals, such as pigs and birds, also enables the virus to change itself much more abruptly. When two types of the A strain infect a single animal at the same time, the viruses can swap genes and emerge completely different. This is exactly what happened in 2009, when two viruses combined into a new one and emerged from pigs to infect humans, causing the swine flu pandemic.
What's a cold? How is it different to flu?
A cold is also caused by a virus, and works in much the same way. But the flu is much, much more dangerous, and creates much worse symptoms.
There are more than 200 entirely different virus types that can cause a cold. That explains why we get colds so often – because there are so many different types, and they can all make us sick.
"It is often very difficult to tell which you have. Doctors will use a laboratory test."
Colds and the flu have similar symptoms: a cough, runny eyes, a sneeze, body aches and a general feeling of unwellness.
A flu has all these symptoms – although a runny or blocked nose is more likely if you have a cold – but they are much more intense and last longer. A flu lasts about a week on average after the first symptoms appear but you may feel tired for several more days. A cold generally lasts for a few days to a week.
Because the symptoms are so similar, it is often very difficult to tell which you have. Doctors will use a laboratory test to confirm their diagnosis.
How do colds and the flu get inside you?
When infected people cough or sneeze, they release tiny droplets containing the virus into the air, which you breathe in.
The cells in your throat and lungs are coated with a sugar called sialic acid. Using its glue, the flu can bind to that sugar.
Colds also have glue, and target the same cells in the throat and lungs, although they stick to different molecules.
What happens when the flu gets inside your cells?
It starts replicating. The virus injects its genetic material into a cell and tricks the cell into copying it. Your own cells start to produce large numbers of viruses, which then travel through the bloodstream to infect other cells. Soon your body is filled with hundreds of millions of copies of the virus.
"You could hop on a plane, feel perfectly healthy, but have a high virus … "
But you won't feel sick right away.
“The flu replicates really quick. It would grow to peak within a day or two. This is the irony,” says Professor Stephen Turner, head of Monash University’s microbiology department. “Those first few days, people are feeling fine – but that’s when you have the highest amount of virus. You could hop on a plane, feel perfectly healthy, but have a high virus – and spread it to all those people.”
Why does the flu make you feel so lousy?
Eventually, all these circulating viruses will trigger the immune system. This happens when infected cells send out little distress messages, letting the body know it is under attack.
The first thing that happens is your body triggers a huge inflammatory response. Parts of your body will literally swell up – this is why your joints ache. You will feel hot and cold. Your body starts increasing its internal temperature in a bid to slow the virus’s replication. That’s why you get a fever.
"If this all sounds as if you have been invaded by a nefarious life form, it's because you have."
To get rid of the infected cells in the lungs, your body starts coughing. Mucus pours through your nose in a bid to flush out any viruses living there.
What about sneezing? Sneezing is not caused by your immune system. The virus is actually designed to trigger sneezing, even though it's not necessary, so it can spread itself via the droplets you sneeze.
If this all sounds as if you have been invaded by a nefarious life form, it's because you have. By its nature, a virus seeks to conquer – but then again your immune system is designed to repel the attack.
All up, you end up feeling pretty miserable – but it’s mostly not the virus’s fault. It’s your body’s immune system fighting back.
How does your body fight back?
The inflammatory response is your body’s first line of defence. The second lines are much stronger.
Cells called macrophages swim through the veins and collect samples of the virus, and bring those chunks to your lymph nodes, which sit at various points in the body, including under your arms and at the base of your throat.
In your lymph nodes, you have billions of slightly different B and T cells – literally a handful for every possible virus shape.
B cells produce antibodies that predominantly target a virus’s scissors and glue. The antibodies neutralise the influenza viruses so they become non-functional and tag them for attack by other cells. T cells, on the other hand, are capable of directly killing virus-infected cells.
“Whatever pathogen you come across, there will be a cell there ready to go,” says Professor Turner.
"Together, these cells attack and kill the virus."
The chunks of virus are then matched against the cells, until the exact right ones are found.
These are the cells your body will use to fight back. Once the match has been made, these specific B and T cells multiply to produce an army that can fight the infection.
B cells produce antibodies that predominantly target a virus’s scissors and glue. The antibodies neutralise the influenza viruses so they become non-functional, and tag them for attack by other cells. T cells are capable of directly killing virus-infected cells. Together, these cells attack and kill the virus.
This happens over the course of several days, before the army is ready for battle. This is what causes swelling and pain in the lymph nodes. It’s not the flu – it’s your body’s armies, getting ready for battle!
Together, these cells attack and kill the virus.
How long does this go on for?
Once the infection has been cleared, most of the B and T cell army dies off. But some are left over, and remain circulating through the body or patrolling the airways. This is what gives us immunity to the virus.
The first time the virus attacks, it takes several days for the body to grow an army to attack it.
"We are constantly being infected, every day."
By keeping a few small soldiers specifically trained against that virus, the body can catch it straight away if it comes back.
Those cells can then either kill the virus themselves or quickly regrow a new army. And, in fact, we are constantly being infected, every day – but these cells kill off the virus before the infection gets anywhere.
A typical flu lasts about eight days after symptoms start but symptoms can hang around for weeks.
Can catching the flu kill?
Yes. Worldwide, it kills between 291,000 and 646,000 people a year, according to the US Centres for Disease Control and Prevention. That is why vaccination is so important.
"The very young and the very old are most at risk – all the more reason why everyone should get vaccinated."
But it's not really flu itself that kills. Sadly, in most cases it is the body that harms itself. In some cases, the body's immune response is too strong. So much tissue is destroyed in the lungs by the T cells, as they hunt for and destroy infected cells that the lungs can no longer function properly, depriving the body of oxygen.
In other cases, the immune system overreacts and becomes exhausted, allowing bacterial infections to creep in. The very young and the very old are most at risk – all the more reason why everyone should get vaccinated.
Is man flu real?
The Oxford Dictionary defines man flu as a cold or similar minor ailment as experienced by a man who is regarded as exaggerating the severity of the symptoms.
Essentially, it's when a man gets a cold and moans about it so much, it appears they have the flu.
That said, some science does appear to suggest that men have it worse when they do get flu. Studies have shown that, in general, men develop weaker immunity after flu vaccination, they have a weaker immune response to the flu when they get it, and they get over it much more slowly.
So a flu may, in fact, really be worse for men.
Catriona Nguyen-Robertson is a PhD candidate at The Peter Doherty Institute for Infection and Immunity. Liam Mannix is The Age's science reporter.< Go Back