Four unpleasant things that can happen if you don't wash your undies enough.

We’ve all done it. You’re rushing to get out the door, you can’t find a clean pair of knickers, so you scoop yesterday’s off the floor and pop them on.

It might seem harmless, but if you continually reuse yesterday’s underwear or don’t clean your knickers often enough – things can get a little bit, erm, crusty down there.

According to Dr Brad McKay, GP and former host of Embarrassing Bodies Down Under, reusing dirty underwear can lead to some serious health problems.

"If you're wearing the same underwear for a while - you're going to be building up bacteria and fungus," he told Mamamia.

"Just imagine a layer of crust building up in your groin area," he said. "It's basically like carrying around a cup of nasty bugs near your vagina."

Here's exactly what can happen to your body if you don't change your knickers:

• Your skin will start to itch.

"Simply put, if you don't change your underwear, you'll start to itch," Dr McKay said.

The build up of bacteria in your unwashed underwear will start to irritate your skin and you'll feel itchy - meaning you will have an urge to scratch your groin in public - which is never a great idea.

And that scratching, as Dr McKay explained, could cause you to break your skin and lead to a staph infection in your groin.

• You will develop rashes, fungal infections, ingrown hairs and pimples. 

That build up of bacteria can also lead to a range of skin conditions including tinea cruris (which is basically athlete's foot in your groin), ingrown hairs and an outbreak of pimples.

"With all of this bacteria, your groin is going to start to look and smell different and it may affect your sex life," Dr McKay explained.

• You'll get thrush. 

If the bacteria from your dirty underwear enters your vagina, you can develop thrush or even bacterial vaginosis.

Bacteria and yeast thrive in moist environments, so you're more likely to experience these conditions if you're wearing underwear that you've repeatedly sweated into and haven't washed.

"If you don't change your underwear, you'll experience a change in your vaginal flora and you're more likely to experience a candida infection or even vaginosis," Dr McKay explained.

"These conditions come with some nasty symptoms - including a milky discharge with thrush, and a grey and slimy, stinky, fishy smelling discharge if you develop bacterial vaginosis."

• You could even get a UTI.

Untreated thrush can lead to a Urinary Tract Infection, which will need to be treated with antibiotics.

"A UTI occurs when bacteria crawls up the urinary tract," Dr McKay explained. "And there's more chance of that happening if you haven't changed your underwear in a while."

And if you're wearing dirty knickers, it's likely fecal matter will move from the back of your underwear to the front. Which is horrifying, even if you don't end up with an UTI.

Sam Armytage's underwear is not "news". Post continues...

While wearing the same underwear two days in row probably isn't going to lead serious health problems, going any longer than that could cause you some develop some inconvenient and embarrassing conditions.

To avoid all of this, Dr McKay recommends changing your underwear daily and sticking to breathable cottons. If you do experience any of these symptoms, it's important to treat them straight away so they don't lead to something more serious.

So change your knickers, people. CHANGE YOUR KNICKERS.