In the year since January 13, 2011, India has had zero cases of polio. Previously, India led the world, accumulating over 5,000 cases since 2000. Polio's last victim in India was 18 month-old Rukhsar, a girl in West Bengal who began showing signs of paralysis on this day in 2011. Now, epic immunization efforts have brought global eradication of the disease a giant step closer. Outside India, however, backsliding Pakistan and Nigeria and splotches of polio across Africa have blocked the final stamping out of the disease worldwide.

Population, heal thyself

Humans still compete against disease, in some places, by having large families because so many children die from illness. India's most polio stricken states, Uttar Pradesh and Bihar, are prolific. They have the highest birthrates in a country that ranks as one of the world's most fertile. For years, the inability to immunize the many hundreds of thousands of children born each year in these northern states fueled the circulation of polio throughout India and even led to the export of the disease to countries in Africa where it had been eliminated, Angola being one example.

India's high-risk population is not only enormous but also highly mobile and difficult to reach. Annual flooding of the Kosi River in Bihar, for example, literally blocks vaccination efforts. As many as six million people are on the move each day in India, according to a World Health Organization estimate. Those millions comprise a diversity of groups and tribes, from rag pickers in West Bengal to seasonal migrants from Rajasthan in the east. They sometimes distrust outsiders implicitly and likely won't even tell health workers their names. Over time, India's immunization program built links to the rag pickers and other groups, learning patterns of movement and number of newborn children needing immunization.

The many, locally adapted polio vaccination efforts are rolled up into monumental, bi-annual campaigns. Each campaign now synchronizes the efforts of 4.5 million people, resulting in 172 million children immunized against polio in each campaign.

Vaccine delivery rather than scientific progress has been the key. "The science was there really when we started," said Dr. Robert Scott, chair of Rotary's International PolioPlus Committee. (Rotary has been a key funder of eradication, which became a priority for it following a 1988 resolution passed by the World Health Assembly.) The oral polio vaccine in use today came from the laboratory of Albert Sabin half a century ago. Only in recent years was his trivalent vaccine, active against the three strains of polio, snapped apart into monovalent and bivalent versions, which were found to be more effective in eliciting immunity. Of India's remarkable triumph, Scott says: "It's a miracle."

Pakistan, Nigeria struggle

India's exertions will need to continue, as its population's immunity must be maintained until the rest of the world eliminates polio. But there is some relief: "Actions have already begun to make the effort more sustainable," according to Apoorva Mallya, program officer at the Bill and Melinda Gates Foundation. "There are tangible improvements in routine immunization in Bihar and other states, and aggressive efforts in place in Uttar Pradesh." But, concludes Mallya, "we will still be going at a very intense effort [level] to make sure cases don't come back in India."

Neighboring Pakistan now tops the world in polio cases—and is increasing its lead. Polio worsened last year, with 189 cases, up from 140 in 2010. The rising wave spilled across borders into China, which has reported 21 cases. Pakistan recently devolved the functions of its ministry of health to its five provinces, and the ensuing absence of a central health authority has hampered eradication efforts. Longer term, such decentralization has improved health in other countries, but it can take five years or more. Pakistan's provincial-level leadership for immunization is said to be good already. Indeed, in the more developed and stable provinces of Punjab and Sinh, some believe there is no reason for the continued, low-quality vaccination campaigns which miss significant numbers of children. In the remaining provinces, which have the most polio and greater instability, the vaccination program suffers from local competition for power and patronage

In Pakistan, polio flows back and forth across the border with Afghanistan, where cases are also up. Most arise in combat areas, but more cases are popping up across the country, leading to a million-child increase in the size of an upcoming immunization round. However, "there is no definitive data showing campaign quality has worsened," according to the Gates Foundation's Mallya. Vaccinators on the ground do report increased problems with security but military authorities say the opposite. There have been no incidents of direct arms fire on vaccinators for two years. In an unspoken agreement, NATO and the Taliban try not to fight on vaccination days, and the Taliban provides letters to vaccinators to ensure safe passage. In sum, for both Afghanistan and Pakistan, "Political change and security probably have marginal impacts on overall [vaccination] quality," says Mallya.

In Nigeria, however, recent unrest will negatively impact eradication. The Islamic group Boko Haram exploded a car bomb next to the United Nations building in Nigeria's capital city, Abuja, last August. A number of those killed worked on the polio program. Most of the country's polio afflicts the northern, Islamic states, but continued unrest and heightened security in Abuja, "will limit movement of our technical staff there so we will have less of an ability to closely monitor the program on the ground," according to Mallya.

Polio in Nigeria worsened even before the flare up of violence. As in Pakistan, local patronage issues encumber the drive to beat polio. It's hard to fire someone if you want their vote, and Nigeria held elections for governorships last year. Last week, the Gates Foundation announced that Nigeria's 36 governors have signed up for an "Immunization Challenge." States that vaccinate 90% of their children this year will receive half a million dollars. Bill Gates traveled to Nigeria last October to begin brokering the deal.

As the sole country in Africa with endemic polio, Nigeria has seeded outbreaks across the continent. Niger, for example, which took itself off the endemic list in 2006, continues to experience importations from Nigeria. Outbreaks have shifted around the continent, exploiting pockets of population with low immunity. Polio has even become re-established in countries where it had been snuffed out: Angola, Chad, the Democratic Republic of the Congo, and perhaps Sudan.

The eradication program continues to battle back in the face of a seemingly endless cycle, using new efforts and initiatives. If global eradication comes, the formula in the countries where polio is still present will be different from the one that succeeded in India. But India's example perhaps demonstrates a new level of capacity, with human volition and aspiration succeeding at massive scale.