Roy Seidenberg, a dermatologist at New York University, explains one of the theories behind that blister on your kisser. Other suspected triggers in this mystery: UV light and genetics.\

1/The Virus Slips In – Despite the name, a common cold virus is not the culprit. Blame the herpes simplex virus Type 1. At some point in your life, you probably invited this invader in by kissing an infected person; the mucous membranes in your lips gave easy access.

2/Your Cells Have Been Hacked – After the herpes virus takes its buss ride, it enters your skin cells. There its DNA replicates in the cells’ nuclei; this process enables the virus to spread to more cells. Eventually it infects your nerve endings and then travels to the base of your trigeminal nerve – the ganglion – where it hides out.

3/Antibodies Attack – Your body knows that it’s been infiltrated. The infected cells alert your lymph nodes, which mobilise white blood cells, called lymphocytes, to give the virus a biochemical beatdown. But in this case, your lymphocytes are more like “wimpocytes” – they can only contain the virus, not destroy it.

4/Stress Screws – You Any time you’re under heavy-duty stress, your levels of the fight-or-flight hormone cortisol spike. That’s good if you need to do battle with your boss or outrun your significant other, but it weakens your whole immune system. Your depleted army of lymphocytes is left to stand guard against herpes.

5/Your Lip Goes Boom – Noticing that it’s no longer surrounded, the virus seizes the opportunity to escape. It travels back down the nerve pathway to stage an attack on your skin cells. You feel a tingle on your lip. Soon, you’re staring at a cold sore. Your immune system needs 10 to 14 days to confine the virus to the nerve again.

Cold sores aren’t the only virus you need to be concerned about…

Is an STD Brewing?

Let’s get drunk and fool around! Careful: booze raises your odds of contracting HPV, a study in Sexually Transmitted Infections reveals. Nearly 70% of men who exceed two and a half drinks a day carry the human papillomavirus – that’s about 11% higher than the rate of more moderate drinkers. Even factoring in sex habits, these guys ran higher risks of any HPV infection (by 13%) and of cancer-causing HPV types (by 35%) than men who drank the least. Alcohol, even in moderation, can hamper your immune system, says study author Dr Matthew Schabath. So use condoms and book an HPV vaccination; it’s nearly 100% effective.

On a sweeter note, however, (perhaps not so) your lack of sugar could be causing you to get into fights with your SO.

Gimme Some Sugar, Baby

When your blood sugar plummets, your fuse might shorten. In a new Ohio State University study, people who were told to poke pins into a voodoo doll of their spouse, depending on their level of anger, jabbed more frequently when their blood sugar was lower. They also blasted their mate with louder, longer noises through headphones during a competitive task. “Glucose provides fuel the brain needs to exercise self-control,” says study author Professor Brad Bushman. When you’re short on sugar, your girl is the most convenient target, he says. “Don’t discuss sensitive topics on an empty stomach,” he says. “Do it over or after dinner.”