When it comes to understanding kids' health, illustrations often work the best! In this case, the best way of thinking about a virus is to imagine it like a stealth ninja warrior trying to take down a massively fortified city.
When you aren't looking, they manage to get on you.
The two main ways cold viruses get onto you is through being picked up from somewhere on your hands or coughed or sneezed onto you.
They sneak past your defences - like nose hairs and sticky snot - and go and find a cell in your body that they like.
Where do they go?
Cold viruses like to live in your nose and throat the best, as your nose is a bit warmer than the outside, but a bit cooler than deeper inside your body.
Just like Goldilocks, they find the place that is 'just right' for them.
Your nose does a great job of getting the outside air ready for your lungs. It makes the normal outside air warmer, wetter and cleaner (whatever the weather/humidity/pollution level) so that your lungs can do their job of taking in oxygen and getting rid of carbon dioxide as efficiently as possible.
But your lungs, being deeper inside you, are too hot for cold viruses, so they just can't go that deep!
What do they do once they are in?
Once the viruses have infiltrated, they start going about their stealth mission... making as many new viruses as possible.
They find a cell, break in and hijack it.
They force it to stop carrying on with its normal business (of say, being part of your nose or throat...) and force it to become a virus factory, making as many new viruses as it can.
Eventually, it gets so full of virus clones that it bursts! Sounds crazy, but yes it really does happen like that. Think mini mini Jurassic Park -in your nose.
Then all the new free viruses can go and find other cells to hijack... and so on until your body realises what is going on and launches the counter-attack.
How quickly do they do it?
It normally takes about 2-3 days (1) before there is enough virus in your system to start to cause a bit of damage and get your body's attention, which is why it can take a couple of days from having that snotty play-date to the inevitable...
That's when the real fun begins... and funnily enough, the things we experience (symptoms) when we have a cold are actually more to do with what our body is doing to fight the infection than the effects of the infection itself.
It's not so much the ninja that makes us feel so ill, its the battle itself! See our other articles for more about this...
So how does this help us?
Well, the thing about cold viruses is that once they are in past your defences, it's basically down to your body to battle it out, there's not a huge amount you can do... so it makes sense to focus your efforts on stopping them from getting in in the first place.
Check out our preventing cold viruses article right here.
(1) Prevention and treatment of the common cold: making sense of the evidence
G. Michael Allan, Bruce Arroll. CMAJ Feb 2014, 186 (3) 190-199; DOI: 10.1503/cmaj.121442