Everyone from Rihanna and Paris Hilton to Justin Timberlake and Brad Pitt has been photographed with cold sores. There's no shame in that.

According to the Mayo Clinic, "about 90% of adults worldwide — even those who've never had symptoms of an infection — test positive for evidence of the virus that causes cold sores."

Why you get a flare-up (usually at the worst possible time) can seem mysterious, however.

Now researchers from the University of Virginia School of Medicine have figured out exactly what goes on inside of you that triggers the appearance of those sores around your mouth or on your lip.

Chronic stress, a sugar-loaded diet, fever, even sunshine can amp up the inflammatory response in your immune system, causing a sequence of reactions that excite the nerves that harbor the sleeping virus. Those aroused nerves then send the infection out into your body to do its dirty work.

In short, your immune system, where inflammation is produced, can become an accomplice to the outbreak when any extra nudge (like stress) that increases inflammation gets your nervous system involved.

If you want to tamp down cold sore flares, it's smart to keep chronic inflammation under control. The best ways are to practice stress-relieving activities like aerobics, meditation, yoga, or tai chi; avoid inflammatory foods like added sugars, red and processed meats, and ultra-processed foods; and promote a healthy gut biome with a plant-based diet, a daily dose of walnuts, and moderate alcohol intake.

That'll keep you smiling.