A woman in the US is suing a cosmetics store because she claims that she caught herpes from their lipstick tester.

In case you're wondering if this is even possible, as a microbiologist, I can tell you that it most certainly is. And it's not just herpes that can lurk in makeup.

But let's look at herpes first.

This very common virus is mainly spread by skin-to-skin contact, kissing and sex, but it can also be transmitted in droplets of spit left by an infected person on towels, cups, cutlery and, yes, lipstick.

Globally, it's estimated that 67% of people are infected with the Herpes simplex virus (HSV-1). But it's a hidden enemy -- a person doesn't need to have visible signs of the virus to spread the infection. The virus lives in facial tissues where it can shed and spread.

It doesn't always show up on the skin as a blister immediately after it has infected a person, instead it can stay hidden and appear after a few months.

Because of this, it's impossible to say with certainty if the American woman caught herpes from this particular store tester, or elsewhere.

Herpes causes blisters on the lips and around the mouth that can last up to ten days. Lipsticks and makeup brushes that touch these parts of the face can then spread the infection to other people.

Fortunately, herpes is a fragile virus and typically only survives outside the body for ten seconds. But it can survive longer in warm and moist environments, such as in sweat. It can also survive between two to four hours on plastic, chrome and water, so there are lots of ways the virus can spread.

There is no cure for a herpes infection, although treatments are available that will reduce the length of infection.

An old problem

Microbiologists have known about the link between beauty products and pathogens since the mid-1940s when talcum powder contaminated with Clostridium tetani bacteria caused the death of a newborn baby

Since the 1960s, contamination linked to other opportunistic pathogens, including Salmonella, Klebsiella pneumoniae and Pseudomonas aeruginosa , has been reported by microbiologists across the globe.

Cosmetic products contain preservatives to help slow down the growth of microbes, but they can become contaminated if people use non-sterile applicators or fingers to apply products, or if the products are poorly handled and stored; for example, products stored in warm and humid or damp conditions such as the bathroom.

Makeup brushes also have the potential to act as suitable homes for bacteria to thrive. Often, beauty blenders and brushes are dampened to help the application of eyeshadows or foundation. But this environment has the potential to promote rapid bacterial growth.

In 2015, it was reported that a 27-year-old Australian woman became paralysed after contracting an MRSA infection that attacked her spine. She used a brush to apply makeup that belonged to a friend who had a Staphylococcus infection on her face, and became contaminated with the organism.

Staphylococcus is a common bacteria that doesn't typically cause harm and lives on the skin or in the nose. However, MRSA (Methicillin-resistant Staphylococcus aureus) is an antibiotic-resistant strain of Staphylococcus.

Irreversible blindness

Mascara wands and eyeliners not only beautify the eyes, but can cause irritation and conjunctivitis, caused by both bacteria and viruses, including herpes simplex.

Studies have revealed that 43% of eyeliners and mascara wands contain contaminants. I know many friends who have been left with a "pink eye" after sharing mascara or using ancient mascara that has been sitting in their makeup bags.

Symptoms of pink eye include redness, watery discharge and, in extreme cases, irreversible blindness.

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Lashes aren't just there to make us look pretty, they serve to block out dirt and bacteria, and using products on the eyes can cause contamination of the product. With time, bacteria can build up in the cosmetic container increasing the risk of eye infection every time the product is used.

If all of these horror stories tell us one thing, it's stay away from sharing makeup products, both among your friends and especially in cosmetic stores. You just don't know what infections other people could be carrying on their skin.

Always keep your own cosmetic products clean and use single applicators where possible. You wouldn't share your toothbrush with strangers, so why make an exception with your cosmetics?