PORT MORESBY, Papua New Guinea — Polio was vanquished by the Pacific nation of Papua New Guinea 18 years ago. Now, as world leaders gather there for the Asia-Pacific Economic Cooperation summit meeting this week, polio has returned — on top of raging drug-resistant epidemics of tuberculosis, malaria and H.I.V., and deadly flash points of preventable diseases like whooping cough and measles.

All over the country, there are symptoms of a profound public health emergency; young and old are getting sick and dying unnecessarily, while facilities lack basic medicines and equipment.

Doctors and experts say the unfolding crisis is the realization of their worst fears after years of deterioration and neglect.

“We were expecting something like this,” Dr. Anup Gurung, a public health specialist with the World Health Organization, said of the polio outbreak at a news conference in the capital, Port Moresby, in September.