It's flu season in the US, meaning that over the next few months, roughly 200,000 people will call in sick to work because it hurts to be alive. But what dictates your personal likelihood of becoming one of those snot-filled few?
While it's generally down to who gets exposed to the bug, new research suggests that you might be able to tell just how bad the flu will be for you personally this year based on the year you were born. So at least you'll know just how awful life with become once the achiness sets in.
This might seem like common sense - of course older people and those with weakened immune systems will respond worse to the flu - but age has nothing to do with it.
Instead, it's all about what strain of the flu you were first exposed to in your youth, because it better prepares your body to fight it again later in life.
"It's not the age, it's the birth year that matters," one of the researchers, Michael Worobey from the University of Arizona, told Hannah Devlin at The Guardian.
"It's breaking new ground for flu, where predictions are really hard. For any given potential pandemic virus, we can actually now say … this is the age group that you can expect is going to end up in hospital dying and this is the age group who will be protected."
Before we go further, let's briefly talk about influenza A. There are many strains of flu that stem from different combinations of the protein hemagglutinin (H) and the enzyme neuraminidase (N). You'll recognise these letters from the name H1N1, which caused panic as it spread back in 2009.
Despite all these strains, for humans they can generally be broken down into two subtypes – or two branches on the influenza family tree.
There's what we can refer to as type 1 viruses, which include the H1, H2, and H5 strains. And then there are the more recently evolved type 2 viruses, which include the H3 and H7 strains.
Until 1968, only type 1 was floating around. Then type 2, which likely evolved from type 1, started making the rounds, dominating from 1968 to 1979, at which point, both of the viruses started to interchange, with one dominating one year and the other the next – though the pattern isn't quite so predictable, reports Devlin.
After analysing historical medical records dating back to 1918, the team saw that whatever one of these two flu types a person contracted early in life left them with long-lasting protection against that type, and susceptible to the other type later on.
So if you were to contract type 1 first, type 2 would hit you harder each year it was dominant.
Worobey uses an analogy to explain this, referring to type 1 strains as 'blue lollipops' and the more recent type 2 strains as 'orange lollipops':
"In this analogy, let's say you were first exposed to a human 'orange lollipop' flu as a kid. If later in life you encounter another subtype of flu virus … that your immune system has never seen before but whose proteins also are of a similar 'orange' flavour, your chances of dying are quite low because of cross-protection.
But if you were first infected with a virus from the 'blue lollipop' group as kid, that won't protect you against this novel, 'orange' strain."
While this discovery might help those who routinely come down the flu to better prepare themselves, the research could also help the world prepare for strains of bird flu.
In fact, the scientists even took their research a step further by examining this link, analysing two different variations of bird flu - H5N1, which is a type 1 flu, and H7N9, a type 2 strain - and then compared reports on how patients reacted to whichever one they contracted.
It turned out that people who were first exposed to a regular non-bird type 1 flu in their youth were 75 percent less likely to need hospitalisation if they contracted the related bird flu (H5N1), and their chances of dying from the illness dropped by a remarkable 85 percent.
This means that our early experience with the flu might help us combat some of the more lethal strains out there that sprung up from time to time in recent memory, though understanding how to stop bird flu from jumping from birds to humans it is still being figured out.
Another important area that these new findings might help is vaccine limits.
Creating a vaccine is not an easy process, which in times of crisis can lead to shortages. Having an understanding of who is better off fighting certain types of flu makes it possible to first administer vaccines to those who are more threatened.
The team's work was published in Science.