You accidentally leave the screen door open, and tragically realise — the mozzies are back.

Unprepared, you're left chasing them around the room clapping at them, terrified that soon they will be snapping at your ankles.

We know some people should be more scared than others, but why are those blood suckers selective in who they bite?

Well, with up to 400 chemical compounds on human skin that could play a role, there's many different answers.

To narrow it down, we've teamed up with mosquito expert, Dr Cameron Webb, a clinical lecturer at the University of Sydney and Principal Hospital Scientist with the Department of Medical Entomology at NSW Health Pathology.

What attracts mosquitoes?

It all starts with carbon dioxide. ( Supplied: Stephen Doggett, NSW Health Pathology )

According to Dr Webb, we first need to understand that the hundreds of species of mosquitoes are attracted by different things at certain distances from you.

"But only females bite; they need a nutritional hit to develop eggs," he said.

So, what makes you so attractive to the ladies?

It all starts with the carbon dioxide we exhale.

"As a long-range attractant, mosquitoes can smell the carbon dioxide we breathe out, and if you think about it, that's the one common denominator between warm-blooded animals — whether it's a bird, a cow or a kangaroo," he said.

Once she gets closer, a mosquito will respond to the smell of the potential blood source's skin, and what's a big contributor to that? Sweat.

"Research of lactic acid in sweat shows it's a key mosquito attractant, particularly for a human-biting species," Dr Webb said.

But what about when you're not sweaty?

While being sweaty could boost your desirability for a mosquito, simply having a shower won't prevent you from getting bitten.

There are many factors, including the bacteria on your skin, that makes you smell different that attracts a certain mosquito.

"It's like going into a bar and choosing between different cocktails; some like a margarita over a mojito," he said.

What can you do about it?

Aside from repellent, not a great deal unfortunately. That is, unless you spend some time and do your own experiments.

"If you can isolate a substance, or mixes of substances, that could help you find the perfect lure to use in mosquito traps. You could then say goodbye to topical insect repellents altogether!" he said.

Until then, you can wear loose clothes and if you're outdoors, use a mosquito coil.

Dr Webb said there is no evidence anywhere that proves there's something you can eat or drink to stop being bitten by mosquitoes.

"No, not even eating garlic, or swallowing vitamin B supplements," he said.

You might want to rethink those old wives' tales your grandma told you over dinner.

Are others really getting away without a scratch?

If some people are telling you they aren't being bitten, it might be because they just have very little reaction to them.

"When a mosquito finds the person or animal to bite, she will inject saliva and we all react to that saliva differently, the same way we react to chemical or food allergens," Dr Webb said.

"People who don't react badly may think they haven't been bitten as much as their friends, but they actually have."

The takeaway message

This complacency can be problematic in the spread of diseases such as malaria and Ross River fever.

"Even if you don't think you're a magnet for mosquitoes, you still need to take precautions to avoid bites, especially when you're outdoors during summer," Dr Webb said.