A trip to the beach turned Brooklyn McCasland’s life upside down after she contracted a sight-threatening eye infection while swimming with her contact lenses on.

It happens when a parasite that can be found in water invades the eye, damaging the cornea, causing severe pain and potentially leading to blindness. The condition, named Acanthamoeba keratitisit is rare in the US, but when it does strike, it most often affects contact lens wearers.

McCasland, 23, who has lost sight in her right eye and is awaiting a cornea transplant, now has to spend her days in a dark room due to extreme eye pain and sensitivity to light. She says she wants to spread awareness about the rare eye infection.

“I’ve been going to the eye doctor since I was 7 years old, getting checkups every year, and not (during) one of those visits have they ever said not to swim or shower in my contacts,” she tells TODAY.com.

“I think there are still a lot of people who don’t know, like me.”

Contact lenses and water do not mix

McCasland, who lives in Longview, Texas, has been wearing contact lenses for 16 years. She says she has always practiced good contact lens wear habits, including changing the solution as directed and never sleeping with the lenses on.

She wears contacts for months, which means she takes them off every night and replaces them with a new pair after a month of wear.

In late August, McCasland went on a girls’ beach trip to Alabama and swam underwater in the Gulf of Mexico while wearing her contacts.

Contact lenses and water “make a bad combination” because water makes contact lens wearers vulnerable to eye infections, Centers for Disease Control and Prevention warns.

People should never wear contact lenses while showering or swimming, the agency notes.

But like many contact lens wearers, McCasland had never heard that warning.

About a week after she returned from the beach, she began to feel that something was in her right eye. That sensation turned into sharp pain a few days later. McCasland went to her eye doctor and got antibiotic drops and other treatments, but nothing helped.

It would take weeks to get the correct diagnosis: Acanthamoeba keratitis.

What is Acanthamoeba keratitis?

Acanthamoeba are single-celled organisms that can be found in swimming pools, hot tubs, tap water and shower water, according to American Academy of Ophthalmology.

They can invade the eye through a minor abrasion of the cornea and infect it, causing severe pain, inflammation and even blindness, notes the CDC.

Soft contact lenses can act like a sponge, absorbing water and impurities from it, such as Acanthamoeba, and becoming their portal of entry into the cornea, Dr. Thomas Steinemann, clinical spokesman for the American Academy of Ophthalmology, TODAY.com previously said.

Contact lenses can also create small cuts on the eye that make it easier for the parasite to enter as they get trapped under the lens, says Dr. Danielle Trief, assistant professor of ophthalmology at the Edward S. Harkness Eye Institute at Columbia University Medical Center. in New York, TODAY.com previously said.

There are one to nine cases of Acanthamoeba keratitis per 100,000 people, with up to 93 percent of cases occurring among contact lens wearers, according to the study. National Library of Medicine.

Because it is so rare, Acanthamoeba keratitis is difficult to diagnose. McCasland says she was misdiagnosed for a month.

“You feel like you have glass in your eyes”

Her eye doctor eventually referred her to a cornea specialist, who quickly suspected Acanthamoeba keratitis. Confirmation required scraping cells from the eye with a metal tool.

Treatment involved putting chlorhexidine and brolene antimicrobial eye drops into the eye every 30 minutes day and night, and then from 6am until midnight. She may drop to four times a day soon.

He also takes Impavido, an antiparasitic medicine, three times a day.

McCasland has been unable to work at her job as a barista in a coffee shop for more than a month due to severe eye pain.

“It’s like you have a glass in your eye,” she says. “I don’t think people understand how intense the pain is. It’s really debilitating.”

Her medicine comes with side effects that make her sick. He also suffers from headaches caused by inflammation of the cornea, and suffers from nerve pain that goes up the right side of the head and into the cheek bone. Pain medication helps only a little, she says.

Because she is extremely sensitive to light, McCasland keeps her eyes closed as much as possible and stays in her bedroom with blankets covering her windows for added darkness. He only goes to doctor’s appointments. She can’t drive, so her boyfriend takes her.

A cornea transplant will restore vision in her right eye, but that may not happen for a year – until her doctor is sure the infection is completely gone from her eye.

“I am very sociable. I like hanging out. So for me to sit in a dark room all day … it was super, super hard,” says McCasland.

“I’m still looking at a long road to recovery. But I wanted to spread awareness for this.”

McCasland’s friend set up a GoFundMe page to raise money for her expenses as she recovers.

How to avoid infection:

The CDC has these tips:

• Do not swim, shower or use hot tubs while wearing contact lenses.
• If water gets on the contact lenses for any reason, remove them as soon as possible. Throw them away or clean and disinfect them overnight before wearing them again.
• Do not use tap water to store or disinfect contacts.

This story first appeared on TODAY.com. More from today: