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Ever wondered why your best friend’s bad mood rubs off on you so easily?

We’ve got the answer…

Obesity
Your chance of becoming obese increases by 57 percent if your best friend is overweight.

According to the docs at Harvard Medical School who found the link, we’re more likely to relax our own behavior if friends gain weight, and that can lead to us piling on the pounds, too.

What to do: Healthy habits are also contagious. It’s been proved that dieters who surround themselves with slimmer friends are more likely to succeed. “It’s a great idea to spend more time around people who practice a healthy lifestyle if you’re trying to change your own eating habits,” says Harley Street dietitian Noor Al Refae. “Ask them for ideas on meals, or simply order the same food they do when you go out to eat. Or, at the very least, let them order first. This has been shown to increase the chance of you making a healthy choice.”

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Immunity
New research shows that when we live with a partner, we “catch” elements of their immune system by kissing, eating the same foods, or exposure to local germs.

What to do: Nothing, it’ll happen naturally. Immunologist Dr. Adrian Liston, of Belgium’s VIB research institute, says: “If you live in a city – where infections come from viruses – but move in with someone who lives on a farm, where parasites are more of a problem, wouldn’t it make sense for your immune system to get attuned to parasites?”

Stress
Feeling under pressure?

The person sitting next to you could be to blame.

According to research from the Max Planck Institute in Germany, simply seeing someone else stressing out causes the levels of stress hormone cortisol to rise in your own body.

What to do: “If you feel signs of stress, such as irregular breathing or rapid heart rate, focus on your breath,” says psychologist Megan Arroll from BPP University in Manchester. Breathe slowly and deeply. As you inhale, fill your lungs so your tummy rises. As you exhale, your tummy needs to fall. “Breathing this way activates the diaphragm, which switches on the part of the nervous system that negates the stress response,” says Megan.

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Hangovers
Alcohol consumption is “catching” – in fact, researchers at Canada’s Dalhousie University could predict a person’s future alcohol consumption simply by looking at how much their partner drank.

This may be because we like to please our other halves.

“But also, the more you’re exposed to the binge-drinking habits of your partner, the more positive your image of it becomes,” says alcohol researcher Sara Bartel.

What to do: Unfortunately, Sara says that trying to change your own behavior isn’t likely to help unless your partner changes, too. So it might be time to plan a few nights out for the two of you at the cinema, or other venues that aren’t based around booze.

Rudeness
If someone is rude to you, it’s likely you’ll be rude to the next person you talk to.

Someone being impolite to us primes our brain to look for further negativity, meaning we might even think a compliment like “nice dress” has a rude connotation behind it.

What to do: “Rudeness is rarely a personal attack, although it may feel like it. So if someone is ill-mannered to you, take a moment and look at what might really be behind their reaction,” says Megan. “Then refocus your mind to break the cycle. Either use mindfulness to change your focus – spend time looking at a nearby object in minute detail, or simply repeat positive phrases in your mind to reset your thoughts.”

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Feeling cold
Brrr – research from the University of Sussex has found that watching someone shiver causes a drop in your body temperature as well.

Apparently, it’s a throwback to when we lived in tribes and tuning into each other’s feelings could have been the difference between life and death (thankfully, now it just makes us a bit chilly).

What to do: Putting on more layers is the obvious solution, but here’s a more fun one – pop on a playlist of tunes from your glory days at uni, or do anything else that makes you hark back to the past. Nostalgic emotions stimulate feelings of comfort and actually makes us feel physically warmer, according to Dr. Tim Wildschut from the University of Southampton.

Diabetes
If your partner has Type 2 diabetes, you are 26 percent more likely to be diagnosed with it yourself. Why?

“Simply because we tend to share behaviors like food choices or exercise habits with our spouse, which may increase our own diabetes risk,” says the study’s author, Dr Kaberi Dasgupta from Canada’s McGill University.

What to do: Team up with your partner to lower the risk, says Dr. Dasgupta. “Prepare meals at home instead of eating out or ordering takeaways. Exercise together so you’re more likely to stay committed, and try to avoid stress – which can also increase blood sugar levels.”

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Happiness
Hooray – it’s not just negative things that we can catch, happiness is also contagious.

According to Dutch scientists, happy people excrete chemicals in their sweat that have the power to change the moods of those around them.

What to do If you feel down, spend time with the most joyful person you know. “Seeking out happy people is a vital part of being happy yourself,” says hypnotherapist Tom Fortes-Mayer from Zoë Clews & Associates. “Happy people tend to be more spontaneous, playful and positive, and this makes them uplifting to be around.”

First published on The Sun.