Forty million people worldwide live in total darkness and 90 percent of them live in the developing world.
However, three out of four such cases are reversible, according to Dr. Geoffrey Tabin, director of the division of international ophthalmology at the John A. Moran Eye Center.
Tabin spends a large part of the year traveling around the world restoring sight to thousands of people with cataract blindness.
ABC News joined him on a recent trip 8,000 miles from Tabin's home in Park City, Utah, to Ethiopia, which has one of the highest rates of blindness in the world.
“”Cataract blindness is an epidemic in Ethiopia, and doctors point to poverty, poor nutrition, genetics and the scorching sun as reasons for the devastating numbers.
When the ABC News team arrived at the Quiha Zonal Hospital in the remote city of Mekelle, Ethiopia, there were already hundreds of patients waiting for Tabin.
For the patients, the journey to Mekelle was a pilgrimage of sorts.
They came from cities, villages and small farming communities throughout Ethiopia to see Tabin. Getting to the clinic was no small feat considering their condition. Many had traveled for days.
Cataract blindness is an epidemic in Ethiopia, and doctors point to poverty, poor nutrition, genetics and the scorching sun as reasons for the devastating numbers.
Upon their arrival, each patient was screened, prepped by a nurse and marked with a small piece of tape with a number above their eyes. As part of the testing, the nurses held up fingers in front of the patients' eyes -- most of whom stared ahead with blank faces.
Tabin said seeing the line of patients waiting for his help was powerful.
"It's daunting, but also exciting. You know, when I'm operating, every single eye is a life," Tabin said.
Some of the patients were young children, including 8-year-old Mahlik, who had cataracts in both eyes.
"He can't see the blackboard. He can't see to study," Tabin said, "and we'll be able to help him completely."
Tabin travels light. All of his equipment fits in just one yellow duffel bag.
He and his partner, Dr. Sanduk Ruit of Nepal, have perfected this amazing sight-restoring surgery, with the goal of curing preventable blindness around the world.
The two of them have sought out the blind all over the developing world.
They founded the Himalayan Cataract Project and, together with their teams, have performed more than 500,000 surgeries worldwide, restoring sight to children and adults who otherwise would be forced to live in darkness.
A recently released book, "Second Suns: Two Doctors and Their Amazing Quest to Restore Sight and Save Lives," by David Oliver Relin, chronicled the doctors' work in the Himalayas and Sub-Saharan Africa.
The simple surgery takes just seven minutes and costs only $11.
The doctor makes a quick incision, removes the cataract in one piece and puts a new lens in its place.
ABC News followed the doctor and his team for three days. During that time, every single bed in the operating room was occupied and lines stretched around the hospital.
Patients left with bandages over their eyes and returned hopeful the next day.
"These are all patients that were completely blind, where they could not see the shadow of their hand move this close to their face one day ago," Tabin said. "These people haven't been able to see in up to 10 years, and they're going to be seeing again for the first time."
Patients were lined up early around the hospital waiting for the doctor to remove their bandages.
"I love that delayed moment because, when the patch comes off, it takes a couple of seconds to register, and people realize they can see," he said.
Once the patients realized the change, there was an eruption of sheer joy, cheering, dancing, hugging and kisses for Tabin.
Parents who hadn't seen their families in years got their first look at their children. One woman called her son "handsome," but told him he needed to shave. A priest who had not seen his congregation in years called Tabin a saint.
Tabin said the surgeries restored vision well enough that the patients could pass an American driver's test on that first day.
The International Agency for the Prevention of Blindness estimates that 40 million people in the world are blind. Cataracts are the leading cause of blindness worldwide.
When asked whether cataract blindness could be cured, Tabin said it could.
"This is the one intervention we do, and then people can see [for] the rest of their life," he said.
Through the Himalayan Cataract Project, Tabin and Ruit train doctors, and establish facilities for eye care and treatment, with one goal in mind -- restoring the gift of sight. On this trip to Ethiopia, the doctors treated nearly 900 people.
"It's kind of exciting to actually help this many people in this short amount of time for this little money," Tabin said, calling it, "one of the biggest miracles you can give in medicine."
That gift changed the future for those patients, including 5-year-old Zalalia, who traveled 200 miles with her father to see Tabin.
The day of her surgery, which would remove the cataract that rendered her completely blind in her left eye, she shrugged when asked whether she thought the doctor would be able to help her.
The next day, she was all smiles when her bandage was removed and she was finally able to see. Her father thanked Tabin for giving his young daughter a future.
Click here to donate to Himalayan Cataract Project.