He's almost 90, but that hasn't stopped America's 39th president from continuing to fight against diseases around the world. And one of Jimmy Carter's most passionate fights is against a disease that no longer even exists in this country, but still affects more than 20 million people worldwide - trachoma.

Trachoma is the world's leading cause of preventable blindness. It is spread by houseflies that carry bacteria and deposit it near the eyes. The disease can then be spread even more when discharge from infected eyes winds up on shared towels or on hands. When trachoma infections are left untreated, the eyelids begin to turn inward and scrape the cornea- causing blindness.

When Carter was a boy, trachoma was a concern in the U.S., particularly in the rural area of Georgia that Carter called home. But while it was eradicated in the U.S. in the 1970s, it is still a threat to the more than 320 million people worldwide who live in villages where the disease easily spreads.

"When you go to a village, quite often from a distance you see children and think they're wearing eyeglasses," Carter said in an interview with Reuters. "And you get close to them and you see instead of eyeframes, it's a circle of flies around their eyeballs sucking out moisture. That causes the disease and they don't even know that they should wash their faces."

Thanks to Carter's efforts, and donations of medication from Pfizer, the Carter Center hopes to completely eradicate the disease by 2020.

Carter talks about trachoma and the Carter Center's progress in this ABC interview with George Stephanopoulos:

Jimmy Carter works to end blindness spread by houseflies in developing countries
The Carter Center hopes to completely eradicate this form of preventable blindness by the year 2020.