Reaching the milestone is a testament to the persistence of the WHO, the Centers for Disease Control and Prevention, the United Nations Fund for Children, Rotary International, the Bill & Melinda Gates Foundation and the United Nations Foundation.

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It has been one full year since polio was detected anywhere in Africa, a significant milestone in global health that has left health experts around the world quietly celebrating.

The last African case of polio was detected in Somalia on Aug. 11, 2014, the final sign of an outbreak with its roots in Nigeria — the one country where the virus had never been eradicated, even temporarily. But the last case in Nigeria was recorded July 24, 2014.

Africa has never gone so long without a case of polio. But in an indication of how nervous experts still are that the disease may resurge, even the announcement from the Global Polio Eradication Initiative was tentatively headlined “Is Africa Polio-Free?”

“This is a big success, but it’s still fragile,” said Dr. Hamid Jafari, the initiative’s World Health Organization (WHO) director. “There’s always a worry that there could be an undetected case in a population you’re not reaching.”

Reaching the milestone is a testament to the persistence, deep pockets and adaptability of the eradication initiative, which is led by the WHO, the Centers for Disease Control and Prevention, the United Nations Fund for Children, Rotary International, the Bill & Melinda Gates Foundation and the United Nations Foundation.

When the global polio-eradication drive began in 1988, more than 350,000 children around the world were paralyzed by the virus each year. Last year, only 359 were.

The case count has been below 2,000 annually since 2001, and eradication efforts now cost about $1 billion a year. But to the frustration of epidemiologists, the virus is a master of the cross-border jailbreak. Thirty-four cases have been found this year, all in Pakistan or Afghanistan, the last place in which the virus is known to persist.

Many scientists now say a worldwide victory over polio is in sight.

“This puts a lot of pressure on Pakistan to do better,” said Dr. Elias Durry, who leads the WHO’s effort in that country and has fought polio in six others, including Somalia and Nigeria.

Usually, Africa is where diseases make their last stand. The last case of smallpox was found in Somalia in 1977, and the last case of rinderpest, a centuries-old cattle disease that may have killed millions of humans by causing famine, was recorded in Kenya in 2001.

Even assuming there are no more cases, Africa will not be officially declared polio-free for another two years. The WHO requires three case-free years because surveillance is difficult in a continent of isolated villages and nomadic herders.

Since several other diseases can cause paralysis, stool samples from each suspect case must be analyzed to definitively exclude polio as the cause.

The Nigerian government was galvanized into action after a new monitoring board began singling out failures in its vaccination program in 2011 and after the WHO declared polio a world health emergency this past year.

Nigeria marked its year without polio July 25 with a modest tree-planting ceremony at which the new president, Muhammadu Buhari, was photographed putting vaccine drops into the mouth of his infant granddaughter.

It was subdued because the campaign “did not want to send out the wrong message to political officeholders and donors that polio has been eradicated,” said Dr. Faisal Shuaib, a health-ministry official who helped lead both the country’s polio-eradication efforts and last year’s Ebola response.

Vaccination and surveillance efforts need to continue, he and other experts emphasized.

Northern Nigeria, homeland of the predominantly Muslim Hausa ethic group, became a polio hot spot in 2003 when waves of rumors began spreading about the vaccine: that it sterilized Muslim girls, that it contained pork products and that it contained the virus that causes AIDS.

The governor of Kano, a northwestern state, stopped vaccinations for a year, and a local polio strain spread, first to 10 other African countries and then, through pilgrims, to Mecca in Saudi Arabia, and to Yemen and Indonesia.

Since then, donors have poured billions of dollars into the eradication campaign, which pioneered many new tactics. WHO scientists provided their Nigerian counterparts with evidence that the vaccine was safe, for instance, and imported more of it from Indonesia, the only Muslim country with a WHO-approved vaccine factory.

Islamic scholars were flown in from as far away as Saudi Arabia to endorse vaccination as halal — religiously acceptable. Bill Gates twice visited the Sultan of Sokoto, leader of Nigeria’s Muslims, asking for his support. Experts from India, which saw its last case of polio in 2011, traveled to Nigeria to share what they had learned.

UNICEF recruited and trained 16,000 “community mobilizers” to fight rumors by talking to their neighbors. Some were themselves polio victims, who displayed their withered limbs as a warning.

Because petty corruption was a major hindrance, seven operations centers were set up around Nigeria where each day’s vaccination progress was publicly tracked through cellphone reports. (The main one, in Abuja, became the headquarters of Nigeria’s successful fight against Ebola last year.)

Surveying teams made detailed maps of villages, squatter camps and other hard-to-reach areas. Vaccination team leaders were handed cellphones that tracked their movements to prove that they had covered their assigned beats. More mothers were hired because they are better at persuading nervous young mothers to vaccinate and because they showed up for work more regularly.