By Jerilyn WatsonThis is Bob Doughty.And this is Sarah Long with SCIENCE IN THE NEWS, a VOA Special English program about recent developments in science. Today we tell about the disease polio. Health experts are working hard to end the disease around the world.

Polio is a serious infectious disease caused by a virus. It usually affects young children. But adults also can develop the disease. It can spread quickly through communities. The disease attacks the central nervous system. It severely damages the spinal cord and muscles of its victims. Polio first causes fever and pain in the head and throat. People infected with the disease have trouble moving their neck and back muscles. Pain also develops in the legs as the muscles weaken. In time, polio victims may not be able to stand or walk. And, some people die from the disease.

Fifty years ago, polio was killing large numbers of people around the world. Some survivors never walked again. Others could stay alive only inside breathing machines. Parents in the United States tried to protect their children by keeping them away from crowds.The situation improved in the Nineteen-Fifties and Nineteen-Sixties. American scientists Jonas Salk and Albert Sabin developed medicines that prevented the disease. These vaccines were given to millions of children. Polio began to die out. In Nineteen-Eighty-Eight, international health workers began a campaign to make the world free of polio.

Experts had hoped this would happen by the end of last year. But reports of new cases have delayed the target date. The United Nations Children's Fund now says it hopes the world will be free of the disease by Two-Thousand-Five. The experts say this goal can only be reached if children around the world receive the polio vaccine.

((MUSIC BRIDGE))The Western Hemisphere had been free of polio for nine years. Then, last year several children in the Dominican Republic and Haiti suffered confirmed cases of the disease. Doctors now suspect about forty-five cases of polio in the area. Scientists at the United States Centers for Disease Control and Prevention blame an unexpected genetic change in the polio vaccine that is taken by mouth.

The vaccine that changed into a dangerous form reportedly was given to children near the affected area about two years ago. It became one of the extremely rare vaccines responsible for causing at least one case of polio. Then the disease spread.

This is because only about thirty percent of children in the area had received the complete series of vaccines to protect against polio. After the polio cases were identified, public health officials in the Dominican Republic and Haiti began giving the vaccines to large numbers of children. Public health officials now say the disease is under control and is not a public health threat.The vaccine that apparently started the new polio cases is the same kind that has prevented millions of cases of the sickness. This oral vaccine has been used for almost forty years. Albert Sabin developed the oral vaccine. The United States approved it in Nineteen-Sixty-One. The vaccine contains three kinds of live polio virus. However, the live viruses are very weak. They produce a reaction in the human body's defense system. The defense system protects against a possible later attack by the polio virus.

However, that is not what happened to the people who got polio in the Caribbean area. Instead, the weakened polio viruses in the vaccine somehow changed. Instead of protecting against the disease, the vaccine apparently caused it.

Experts say this kind of genetic change is extremely rare. It happened once before. A genetic change in the polio vaccine took place in Egypt in the Nineteen-Eighties. At least thirty people there got the disease.Scientists know that weakened viruses in the oral polio vaccine change as they pass through and out of the human body. These changes are almost always harmless. But researchers are trying to answer the question of how unexpected, harmful changes happen. They want to know this so that these changes will not happen again.

Hiromu Yoshida studies viruses at the National Institute of Infectious Diseases in Tokyo, Japan. He led recent studies of genetic changes that might harm the polio vaccine.Doctor Yoshida's team studied waste water and river water in Japan. They did so because weakened polio virus sometimes gets into the water supply. This happens as the virus passes through the human digestive system. The scientists performed their research after many people had taken the oral polio vaccine. They found that a few viruses made abnormal, unexpected changes.

Doctor Yoshida said temperature differences might have caused the changes. The human body is warmer than the temperatures used in preparing the vaccine. But he said his team's research did not prove that the changed genetic structures could cause disease.The risk of harmful genetic changes in viruses in the oral polio vaccine is small. Still, the United States changed its method of giving vaccines against polio. The nation stopped using oral polio vaccine last year. Doctors now give an injected vaccine that does not contain any live polio viruses. The new vaccine is similar to the first polio vaccine developed by Jonas Salk in Nineteen-Fifty-Five. However, the oral polio vaccine is still being used in most countries. This is because it is more effective than the injected vaccine. And it is much easier to give to large numbers of children in developing countries.

((MUSIC BRIDGE))Experts say between ten and twenty-million people around the world are victims of polio. The effects of polio can revisit some victims many years later. This is called post-polio syndrome. The victim can suffer further weakening of muscles damaged earlier by polio.

The syndrome may appear up to thirty-five years after the first polio attack. People with serious permanent polio damage are more likely to suffer this problem. Some patients have only minor problems. Others have more severe effects. Post-polio syndrome can cause weakness, tiredness and pain. In severe form it can also interfere with breathing and swallowing.

((MUSIC BRIDGE))Scientists say polio still threatens about twenty countries, mainly in Africa and South Asia. Seventy percent of people infected with polio live in India. Bangladesh had planned to celebrate the end of polio in that country in December. But an eighteen-month-old child became sick with the disease. So Prime Minister Sheikh Hasina had to cancel the ceremony.

The disease continues even though many countries have increased efforts to provide children with the vaccine. In many countries, medical teams find it difficult to vaccinate children against polio. Sometimes this is because communities are far from cities in areas where travel is difficult. Civil conflicts are a problem in several countries in Africa. Sometimes travel is not permitted or areas are too dangerous to enter.Generally, however, health experts have made good progress against the disease in recent years. The World Health Organization began an anti-polio campaign in Nineteen-Eighty-Eight to vaccinate all young children.

At that time, one-thousand children around the world got polio every day. Today about twenty new cases of polio are reported daily. Sometimes, however, cases are not reported. This creates the danger of the disease spreading widely. For example, Japanese researchers recently found eight unreported cases of polio in Laos.

The scientists worry that this polio could spread to other people. Former American Health and Human Services director Donna Shalala said, "No nation is truly free from polio unless every nation is free of polio."

This SCIENCE IN THE NEWS program was written by Jerilyn Watson. It was produced by George Grow. This is Bob Doughty.And this is Sarah Long. Join us again next week for more news about science in Special English on the Voice of America.