You may want to think twice about showering or swimming with your contact lenses in.

A 41-year-old woman in England nearly lost her vision in one eye from an infection she got after swimming and showering while wearing contact lenses. Her case is described in a brief report published Wednesday in the New England Journal of Medicine.

Many contact lens wearers are unaware of the risk of this infection, called Acanthamoeba keratitis. Indeed, it's exceedingly rare. In the U.S., the Centers for Disease Control and Prevention estimates there are one to two cases per million contact lens wearers.

“I’ve been practicing optometry for 19 years and I’ve seen two cases,” said Dr. Shilpa Register, an optometrist and clinical assistant professor with the University of Alabama at Birmingham department of ophthalmology and visual sciences, who was not involved with the woman's case. “When it happens it's severe, and it can cause blindness pretty quickly if it's not treated immediately.”

The eye infection is caused by a microscopic, free-living amoeba called Acanthamoeba that's commonly found in water and soil. Any kind of water exposure while wearing soft contacts puts a person at risk.

Acanthamoeba keratitis occurs when the amoeba makes its way onto a person's cornea. The infection can result in permanent visual impairment or blindness.

The woman described in the case had been experiencing intermittent pain, blurry vision and sensitivity to light for two months before she sought medical care, according to the report.

To diagnose the infection, doctors take a culture of a corneal scraping. Then, they confirm the diagnosis by putting a dye in the affected eye — if the amoeba is present, the infection will turn from a hazy brown to a bright fluorescent green.

Register said that because of the amoeba's aggressive nature, it's more prudent for doctors to treat the infection immediately if there is a suspicion that a microbe might be the cause of a patient's eye symptoms.

“It’s a difficult infection to treat, and it’s usually aggressive,” she said.

The woman was given antimicrobial drops to treat the infection. However, her vision remained impaired due to scarring and the formation of a cataract caused by the infection. One year later, she had surgery to repair her cornea, which restored her vision somewhat and alleviated discomfort.

In the U.S., the overwhelming majority of people who get the infection — 85 percent — are contact lens wearers, according to the CDC.

Soft contact lenses can act like a sponge, so they absorb water, along with anything found in that water, such as Acanthamoeba, Register said. The contact lens then serves as a portal of entry for these microscopic organisms to the eye's cornea.

“Patients should know about these risks. I give my all patients a do’s and don’ts sheet about proper contact lens use so they know the health risks if they’re not careful,” Register told NBC News.

According to the CDC, all contact lens users should follow these guidelines to help reduce the risk of eye infections:

Visit your eye care provider for regular eye examinations.

Wear and replace contact lenses according to the schedule prescribed by your eye care provider.

Remove contact lenses before any activity involving contact with water, including showering, using a hot tub, or swimming.

Wash hands with soap and water and dry before handling contact lenses.

Clean contact lenses according to instructions from your eye care provider and the manufacturer’s guidelines.

Store reusable lenses in the proper storage case.

If you experience redness, swelling, extra tears or sticky, gooey discharge from your eye, blurry or double vision, light sensitivity, itching, burning, or eye pain, seek immediate medical attention.

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