( CNN ) -- A rare tropical disease called Guinea worm is closer to being eradicated , according to former President Jimmy Carter and other experts . There are now only 542 known cases of Guinea worm left worldwide , as of 2012 , representing a 48 % decrease from 2011 , officials said Thursday at a news conference . `` We can not rest until we get and contain the very last case , '' said Dr. Donald R. Hopkins , affiliated with the Carter Center in Atlanta , which has been instrumental in the effort to wipe out Guinea worm . The World Health Organization this week said in a report that Guinea worm , also called dracunculiasis , has a global eradication target for the year 2015 . So far , only one disease has reached the status of worldwide eradication since 1980 : smallpox . Guinea worm a greater challenge than smallpox . In 1986 , when the Carter Center began leading its international campaign against Guinea worm disease , there were 3.5 million cases in 21 countries . South Sudan has the most known cases today , at 521 , followed by Chad ( 10 ) , Mali ( 7 ) , and Ethiopia ( 4 ) , according to the Carter Center . These are the only four countries that still have transmission of Guinea worm . In Sudan , a war is waged to eradicate the 'fiery serpent ' A main obstacle to eradication is that one case of Guinea worm disease , if not addressed quickly , can spread to hundreds more people , Carter said . Communities that depend chiefly on open water sources like ponds in rural and isolated areas are especially affected , according to the World Health Organization . The disease gets into the body when people drink contaminated water , which contains water fleas that have ingested Guinea worm larvae . The human stomach kills the water fleas , but not the larvae within . Worms can get through the intestinal wall and move around under skin tissues . A patient will develop a painful skin blister , through which a worm will emerge about 10 to 14 months after infection . The worm can be up to three feet long , and looks like a very long spaghetti noodle . When the patient heads to a body of water to try to soothe the fiery pain , the worm deposits more larvae in the water , and the cycle starts over . No vaccines or treatments are available for Guinea worm , but the international effort has made significant strides with health education and encouraging behavior changes . How fight against Guinea worm was won . Prevention strategies include filtering drinking water and discouraging people with Guinea worm from wading in water . Water can also be chemically treated with larvicide . While Guinea worm disease does not usually result in death , it is associated with incapacitating pain and secondary bacterial infections . Patients find it difficult to work , farm and function . In a household where a parent has had Guinea worm , toddlers will be less well nourished because the infected parent will have difficulty farming , Hopkins said . Carter recalled Thursday that he first saw people living with Guinea worm in Ghana . He saw a young woman , about 20 , who appeared to be holding a baby . But he discovered that it was not a baby ; it was her right breast , which was swollen because of the disease . The international effort is monitoring about 7,000 villages , Carter said . `` We need to find anyone who has Guinea worm '' and `` prevent them from going back to the water source , '' he said . In terms of government support from individual countries , South Sudan is most strongly behind the eradication efforts , Hopkins said . In that country , there are about 110 workers on payroll , and some 12,000 unpaid volunteers , Carter said . The program in Mali , however , has not been able to operate fully , or at all , in areas occupied by rebel groups , said Ernesto Ruiz-Tiben , director of the Guinea Worm Eradication Program at the Carter Center . The unrest there has made efforts to stop Guinea worm more difficult . Educating people about Guinea worm has other benefits besides preventing this specific disease , Hopkins said , such as helping people to think more carefully about the water they drink . A clean water supply can help prevent other diseases . Such education efforts also empower local populations , showing them that they can do something to correct their own problems , Carter said . Villagers often want to know how to address other diseases , too . `` The people themselves say , 'How can you help us with other problems ? ' `` Carter said . CNN 's Madison Park contributed to this report .
Guinea worm disease cases worldwide have dropped by 48 % in a year . Score: 6 Only smallpox has been eradicated worldwide since 1980 .