The latest outbreak of polio, which has spread from Nigeria as far as Indonesia, is the result of an untimely confluence of religion, war and waning vaccination efforts in some countries.
Since 2003 -- when the virus started to spread from Africa to the Middle East to Asia -- new infections have been reported in 17 countries that had been declared free of the disease.
Most of those countries have reported only sporadic cases, but Yemen now faces a major outbreak with 63, and Indonesia, which has reported eight cases so far, may be on the verge of a significant flare-up.
Each known case typically represents up to 200 undetected infections, so the virus is far more widely distributed than the statistics indicate.
With the summer approaching -- the high season for polio transmission -- the $4-billion global effort to stamp out the disease, led by the World Health Organization, is facing its most serious challenge since it began its campaign in 1988.
"These are not small setbacks," said Dr. Donald A. Henderson, who led the effort that rid the world of smallpox in 1980. "It's a daunting set of problems they have ahead."
Dr. Bruce Aylward, coordinator of global polio eradication for WHO, said he was confident that the agency would meet its goal of ending polio transmission this year or in early 2006.
The eradication campaign has launched a massive drive to vaccinate 77 million children across Africa to quell the outbreak.
But given the hardiness of the virus, the poverty of countries in which it thrives and the easy flow of people between continents, some scientists question whether it is realistic or prudent to expect that polio will ever disappear."I don't think we can ever stop vaccinating -- or should," said Dr. Mark A. Miller, associate director for research of the Fogarty International Center, a part of the National Institutes of Health. "The sooner we come to terms with that, the better we'll all be."
Despite the latest outbreak, the campaign against polio has been among the most successful in medical history, ranking close to the eradication of smallpox, which is the only major human disease that has been wiped out.
Only a few decades ago, polio was among the world's most feared diseases, a scourge found in nearly every corner of the globe. Spread from person to person or via tainted bathing or drinking water, polio kills by attacking the nervous system. It can cause total paralysis in hours.
In 1954, Dr. Jonas Salk developed a vaccine for polio, followed in 1957 by an oral vaccine developed by Dr. Albert Sabin.
By the early 1990s, vaccination had eliminated polio as a naturally occurring disease from the Western Hemisphere and wealthy nations in other regions where inoculation rates remained high.
The Global Polio Eradication Initiative, in which UNICEF, the U.S. Centers for Disease Control and Prevention and Rotary International participate, initially hoped to eliminate the disease by 2000 but later put off the deadline to this year.
Nonetheless, the global campaign has been very successful, reducing annual cases from a high of 350,000 in 1988 to 1,267 last year. Today, polio is endemic in only six nations: India, Pakistan, Niger, Nigeria, Egypt and Afghanistan.
Nigeria is the epicenter of the current international outbreak. In 2003, officials in the north of the country, a predominantly Islamic region, halted vaccinations after religious leaders voiced suspicion that the vaccine was tainted in a plot to harm Muslim children. Tests by WHO and independent scientists verified that the supply was pure, but vaccinations were stalled for 11 months.
Polio cases, which previously had plummeted in Nigeria, suddenly spiked, and the virus was carried along commercial and cultural routes to other nations.
In 2004, a major outbreak hit Sudan, many of whose people often find work across the Red Sea in Yemen. A guerrilla war in Sudan had further weakened its impoverished people and made it hard for health workers to reach infected areas. Crowded refugee camps made perfect staging areas for the disease.
In January of this year, Sudanese or Nigerians may have carried the virus to Mecca, Saudi Arabia, during the hajj -- the Muslims' annual pilgrimage.
Then Indonesian pilgrims or guest workers apparently carried the disease from Saudi Arabia to Java.
"All of these viruses go back to Nigeria, and they all go back to Sudan," said Dr. David L. Heymann, who directs the eradication effort for the WHO.
In theory, the countries with new infections should have been protected by ongoing polio vaccination drives.