For a brief moment two years ago, Pakistan seemed finally on the verge of defeating polio. One of only two countries in the world where the virus remains endemic, Pakistan recorded no new infections for a little over a year starting in 2021 — the longest virus-free stretch the country had ever experienced.

But since then, polio has roared back, spreading beyond its traditional hot spots to areas once largely untouched by the virus.

Last week, health officials reported the first polio case in the capital, Islamabad, in 16 years. This month, environmental monitoring detected the polio virus in sewage samples from several major cities, including Peshawar and Karachi, Pakistan’s largest city, where millions live in crowded, unsanitary slums.

And the virus has spread to a new epicenter in Balochistan, an arid, restive province in the southwest hundreds of miles from the virus’s former focal point in northwestern Khyber-Pakhtunkhwa Province.

On Monday, Pakistan began a weeklong nationwide polio vaccination campaign involving 286,000 health workers — the largest public health surveillance network in the world — aimed at vaccinating 30 million children under 5. The campaign, taking place across 115 of the country’s more than 165 districts, is part of the government’s renewed billions-dollar effort to contain the spread of the virus.

“I am hopeful that polio will be eradicated in the coming years and months through coordinated efforts,” Shehbaz Sharif, the country’s prime minister, said on Monday. “Polio will be driven out from the borders of Pakistan, never to return.”