CHONGWE, Zambia — On a visit to a public hospital in a farming community in late November, I saw something that astonished me.

Empty beds. Rows of them, their black plastic-covered mattresses stripped of sheets. Blue privacy curtains folded up over rails, out of the way.

I had never seen a Zambian hospital like this. When I last toured one, nearly 15 years ago, patients lay two or three to a bed, head-to-feet-to-head. And more on the floor. More on thatch mats in the hallways. The patients were gaunt, their eyes huge above sunken cheeks. Hopelessness and suffering hung in the air.

Now, the wards were silent, and not just because a Covid-19 wave had recently ended. There was only the bounce of my voice off the walls as I asked Morton Zuze, the doctor giving me a tour, where everyone was. When I told him that I had last spent time here in the mid-2000s, he knew what I was asking.

“Well,” he said matter-of-factly, “there are 200,000 people in this district and 20,000 of them are on ART.” That was a staggering figure: 20,000 people on antiretroviral treatment for H.I.V.

The only sign of H.I.V. that I saw in Zambia was glossy billboards dotted around the capital, Lusaka, showing stylish, smiling people, with the slogan “I’m ending AIDS with …” and some helpful strategy: testing regularly or taking treatment or using drugs to prevent infection.

I was a correspondent based in Johannesburg at the height of the African H.I.V. epidemic, the last pandemic to batter this region. In the early 2000s, there were 28 million people living with the virus in sub-Saharan Africa, and about a third of young adults in Zambia were infected. More than two million Africans were dying of AIDS each year.

H.I.V. had long since become a treatable illness in wealthy nations, but here the price of antiretrovirals was still more than $10,000 per person per year. Entirely, impossibly, unaffordable.

Back then, I reported from villages in Eswatini, then known as Swaziland, where I couldn’t find more than a handful of people my age — just children and elderly people. I wrote from Johannesburg about the day that Nelson Mandela broke a powerful taboo and told South Africans his son had died of AIDS. I told the story of a grandmother named Regine Mamba in Zambia raising 12 orphaned grandchildren. And I interviewed the brave, and often desperately ill, activists, such as Zackie Achmat, a co-founder of South Africa’s Treatment Action Campaign, who were fighting with their lives to get access to treatment.

Almost two decades later, the fruits of what they fought for were vividly on display, and a reminder — useful at this moment as another Covid wave makes this pandemic seem unending — of how much is possible.

Science, in the form of drugs that quelled if not vanquished a deadly virus; a network of fierce, courageous activists; coordinated international efforts, including a massive investment by the U.S. government — they all combined to deliver the miracle of that empty, echoing Zambian hospital ward.

We know how to do this.

In a clinic outside Cape Town, Linda-Gail Bekker, a renowned H.I.V. researcher, mentioned to me almost in passing that “our longevity is back.” When I asked what she meant, she showed me the data: The life expectancy of South Africans, which H.I.V. drove down from 63 in 1990 to a low point of 53 in 2004, has risen steadily since treatment began to be delivered by the public he