One of the foremost killers of young children worldwide is simple diarrhea caused by a virus, the Rotavirus. Rotavirus is highly contagious and can make anyone sick. But babies, especially in the developing world, can become dehydrated and die from the infection. Health experts will gather for an international rotavirus conference in Mexico City from July 7-9 to discuss advances in vaccines that could prevent over half a million child deaths annually.

Health experts dream for the kind of success against rotavirus that they have had fighting other viruses like polio and smallpox. Public health officials eradicated smallpox in the 1970s and they are close to defeating polio. Vaccines are the reason.

"Diarrhea is a huge public health problem. Rotavirus is the leading cause of that and fortunately, we are on the verge of having this vaccine available," said Dr. Jon Andrus of the Pan American Health Organization. The group is a co-sponsor of the Mexico City rotavirus meeting with the U.S. government and the Sabin Vaccine Institute. He says despite the hope for a rotavirus vaccine, the path to developing one has been difficult. A version released in the late 1990s caused an intestinal blockage in some infants. Dr. Andrus says it was taken off the market.

"Now that was a setback," he said. "But we can look at it as actually something positive because when that vaccine was taken off the market, it mobilized a lot of partners to say 'Hey, we still are dealing with this public health problem. What else can we do?' ? New vaccines have been developed that use different technologies, with the data showing that this risk is not increased with these new vaccines."

Drug companies Merck and GlaxoSmithKline have developed new oral rotavirus vaccines from weakened strains of live rotavirus. They are being tested in Latin America, Europe, Asia, South Africa, Bangladesh, and the United States.

The president of the private U.S. National Vaccine Information Center, Barbara Fisher, says that scientists should proceed with the vaccine cautiously and make sure it does not cause problems when given with other live vaccines, like polio.

"It is very important that the pre-licensure clinical trials are comprehensive enough, are conducted in populations with genetic diversity, and that there is a long follow-up period after the vaccine is given to try to detect any serious health problems that could be associated with the vaccine," she said. "The rest of the world, certainly Africa and Central America, South America, do use the live polio vaccine. It is important to make sure that when one adds a new live virus vaccine like the rotavirus vaccine, that it has been studied in combination with the other vaccines given to children."

Dr. Andrus hopes that the experimental vaccines can be ready for governments to approve in a year. He says how widely they would be used after that depends on how much they cost.

"As with most vaccines, when the vaccine is introduced the question is, is it affordable to the family and child that benefit from that technology, and is it also affordable to the company that invests in the research and the resources to make it. Initially it's less affordable for the family? Hopefully over time, however, that price will come down," he said.

Barbara Fisher worries that drug companies might promote the use of their new rotavirus vaccines in the industrial world, where diarrhea is not usually lethal, so that they can make enough profit to offset the cost of providing them more cheaply to developing countries. She cites a health risk to giving children vaccines they don't need.

"It's become a political issue here, because our children use so many vaccines," she said. "Why should we use a vaccine for a disease that's not really a killer in our country? Maybe there needs to be a re-thinking of how we finance getting the very necessary vaccines to the third world."

The Mexico City rotavirus conference addresses the biology of microbe, as well as economics, public health, and vaccine development. Representatives from the Americas, Asia, and Africa are expected to attend.