The staff of the main public hospital here grapples with one epidemic after another. It was cholera a year ago, followed by an outbreak of polio this year. The malaria season is just beginning.

The daily admissions seem to be a catalogue of scourges from a bygone era, diseases that have been all but wiped out in most of the world. But on a recent, bustling weekday at this hospital, one disease was conspicuously absent from the mix: Of the nearly 500 patients, not one has been diagnosed with AIDS.

While Angola's closest African neighbors -- Zambia, South Africa, and Zimbabwe -- struggle with an AIDS epidemic that has infected nearly a quarter of their populations, the deadly virus has infected fewer than 4 percent of Angolans. Instead, this war-stressed country's health crisis centers not on sub-Saharan Africa's newest scourge, but on a succession of anachronistic diseases.

The dichotomy, physicians and health experts here say, is the result of Angola's costly 25-year-old civil war, which has eroded the country's ability to provide sanitized food and water, the primary transmission routes for such diseases as polio.

But simultaneously, the war has essentially quarantined this country of 12.5 million, limiting people's exposure to the outside world and, consequently, to carriers of the HIV virus, which causes AIDS. The irony of Angola's startling insulation from AIDS is that the social disease relies on a civil society to thrive, and with each passing day of war here, civil society contracts.

"The war has fractured Angola and isolated us," said Ivan F. Camanor, UNICEF's health officer here in Luanda, the capital. "Because of the war, the sanitation of food and water has suffered, and that has made Angola an undesirable place for tourists or business people or people looking for work. And few Angolans can afford to leave. So while young, sexually active people go in and out of South Africa every day, that doesn't happen very much here. So Angola becomes an environment for some diseases but not others."

Health officials say that the proportion of the population infected with the AIDS virus was 3.8 percent last year, compared with about 22 percent in South Africa and 25 percent in Zimbabwe. Health workers compiled the Angola figure by sampling expectant mothers, blood donors and people diagnosed with other sexually transmitted diseases, and those populations typically provide a relatively reliable sample to calculate the spread of AIDS.

Still, with Angola's scant resources, and millions of refugees fleeing the fighting in rural areas, medical workers acknowledge that a precise reading may be difficult. Physicians here said that random samples from blood donors over the past few years have revealed HIV infections in as few as 1.5 percent and as many as 9 percent of the donors.

By contrast, the world's largest outbreak of polio in the last decade occurred here this year. The disease, spread by contaminated food and water, killed or paralyzed nearly 500,000 people annually until an effective vaccine was developed in 1955.

More than 1,000 Angolan children were infected with the disease this spring, though an aggressive vaccination campaign by UNICEF and other humanitarian agencies has dramatically curtailed the epidemic. With thousands of refugees fleeing the fighting in their villages and settling in vast squatter camps, with uncollected garbage, open sewage and contaminated water, Angolan cities such as Luanda can be incubators for diseases such as polio and malaria. Angola even has higher rates of leprosy than almost any other country.

But it has so far staved off the spread of AIDS, largely because it is shut off from the rest of the world. South Africa's explosion of AIDS cases coincided with the opening of the country's borders following the country's first democratic election in 1994. The virus has spread in distinct patterns since then, largely along the routes used by truck drivers entering the country, said Trevor Carmichael, professor of medicine at the University of Witwatersrand in Johannesburg.

"Once the disease is introduced, you do need some sort of social structure to spread the disease," he said. "It really depends on this critical mass of people who infect others, and a closed community can delay that from happening."