Guinea worm disease cases were cut to less than 600 in 2012, marking significant progress in eradicating the parasitic infection, former President Jimmy Carter said Thursday.

The effort to eradicate the disease, which is spread by drinking , has been a major focus of the former president and the Atlanta-based Carter Center. Carter said that when the organization began to work on the eradication effort in 1986, there were 3.5 million Guinea worm cases in 21 countries.

In 2012, there were 542 cases reported in four countries, a from 1,058 cases the year before, the center said Thursday during Carter's first social media press conference. was taken off the list of countries with the disease in 2012 after reporting no cases. The only other human disease to be eradicated is .

"When organizing The Carter Center, I have to admit my first projected goal was to bring peace to people," Carter said Thursday. "But then we began to see that the main goal of The Carter Center would be to fill vacuums in the world, that is to deal with things that the or the U.S. government or Harvard University were not doing."

One of those vacuums, he said, is Guinea worm. Carter said the disease often goes unnoticed, even in the countries where it spreads because officials don't know it exists in their remote villages.

The vast majority of cases reported in 2012 were in South Sudan (521), followed by 10 cases in Chad, seven in Mali and four in Ethiopia. Guinea worm can be agonizing, creating lesions on the skin as a female Guinea worm slowly emerges from the body. The only way to treat it is to remove the worm from the body, a process that often takes weeks.

"We are now down to the last few cases, which are the hardest ones in any eradication campaign," said Dr. Donald R. Hopkins with The Carter Center. "We cannot rest until we get and contain the very last case because even one case can explode and set us back."

Carter said significant challenges remain, including security concerns. Dr. Ernesto Ruiz-Tiben, director of The Carter Center's Guinea Worm Eradication Program, said workers have not been able to operate in Mali since last spring, resulting in an inability to detect and contain cases in the former French colony.

A coup in the capital last March has created instability that has allowed Islamist extremists to take control of cities in the north.

The Carter Center's efforts has focused on education. Prevention is key because no vaccine exists. Infected people are kept from entering sources of drinking water and the use of cloth filters is encouraged to remove from drinking water any tiny water fleas that carry the Guinea worm larvae.

"I would say the main obstacle we face now is the fact that just one case of in a village, if not addressed quickly, can be spread to maybe 50 or even several hundred cases the following year," Carter said.