A study by Ashley Madison, the dating website for married people, found that nearly half the members polled had had an affair with a co-worker. Among these cheaters, the majority said they'd had their first encounter at an office Christmas party.

As Peter Saddington, a relationship counsellor at Relate, explains, there are several reasons why affairs are so common at Christmas.

"First of all there's more alcohol, so people are disinhibited and don't respond in the same rational way they do when they're sober. Secondly there's that party atmosphere. People are full of joy, and sometimes you can misinterpret that to mean more than it actually does, especially if home life is difficult," he says.

The emotional impact can be huge

Whatever the circumstances of an affair, it can spell trouble for a relationship. According to Saddington, affairs are one of the most common reasons why people come to Relate. The aftermath can be extremely tough to deal with, for the cheater and for the wronged party alike.

"If your partner's had a one-night stand or is having an affair, the effect can be trauma, that someone you love and trust has done this to you," says Saddington. "Some people feel distressed and emotionally insecure. They might even doubt their sense of self, questioning everything that's come before. Equally, they might start feeling angry and determined to save their relationship."

The person who's had the affair, meanwhile, may be caught between feelings of guilt and liberation.

"It can be a positive experience, that they've been with someone who responded them in a different way to how their partner does," says Saddington. "Or it can be very shaming, and lead to doubts about who they are as a person."

In some cases, being the victim of infidelity can have serious consequences for a person's mental and physical health. The situation has been associated with depresión, ansiedad and unhealthy coping mechanisms such as trastornos alimenticios y substance misuse. Some mental health professionals also believe there can be parallels with trastorno de estrés postraumático (TEPT).

Infidelity has even been associated with heart problems. A varios estudios have found that men who cheat are more likely to have ataques cardíacos, while 'broken heart syndrome' (stress-induced cardiomyopathy) is real - it typically affects older women who have experienced emotional trauma.

Some people are more susceptible to this kind of fallout than others. A estudio reciente, involving 232 college students who'd been cheated on, found that women who blamed themselves for the infidelity were more likely to turn to unhealthy coping strategies. Reactions can also be stronger when people have been through similar experiences in the past.

"If you've had this happen in a previous relationship or one of your parents had an affair when you were younger, the effect can be magnified," says Saddington. "Some people can't come to terms with infidelity because it's linked to an unresolved grief."

He adds that many people who have cheated are in a vulnerable position themselves, and may be contending with loneliness and depression.

"It's important to say that most people don't go out looking for an affair," he says. "Sometimes affairs start out because there have been difficult events at home, and you want someone to talk to. It's easy to slip into something you never intended to happen."

On top of the mental health burden, affairs can have sexual health implications - particularly in the case of a drunken one-night stand that didn't involve condoms. One estudio, which compared people in monogamous relationships with those who had multiple sexual partners, found that both groups were at similar risk of ITS. The researchers concluded that this was due to infidelity.

Since a number of STIs are symptomless, and can have serious consequences if left untreated, it's always best to come clean immediately if you might have put your partner's health at risk.

While an affair is undoubtedly one of the hardest things a couple can go through, it doesn't always mean the relationship is doomed. As Saddington explains, if both parties want to work through their issues, they typically can.

"In many cases the relationship can be stronger as an outcome, because you've faced something really difficult together and have learned to be honest as a result," he says.

In the immediate aftermath of an affair, he feels it is best to avoid involving family and friends, who may take sides and create further complications. Rather, it's important to work out for yourself what you actually want and how you envisage the future. Relationship counselling, via an organisation like Relate, may also help you sift through your feelings.

"Deciding what you do want to do and then talking to your partner about it is perhaps the most important step," he says.