More than 20,000 HIV specialists, patients and activists convened Monday for their worldwide conference, a meeting held this year in the shadow of another virus that causes a deadly new disease with global reach.

The novel coronavirus has disrupted two years of planning. Instead of highlights and challenges, the talk now is of locked-down people with HIV who cannot get treatment, preventive medication or even testing, of lost wages and health insurance. For the physicians and scientists still exploring therapies and a vaccine for AIDS four decades on, some research is threatened or postponed, their patients hunkered down at home.

Modeling conducted by the U.N. Program on HIV/AIDS (UNAIDS) and the World Health Organization predicts that a six-month interruption in health services and treatment medications caused by the coronavirus could result in an additional 500,000 deaths from AIDS and related diseases in sub-Saharan Africa in 2020 and 2021.

“I think this meeting is going to show us the impact of covid-19 in rolling back the progress we thought that we had made in HIV,” said Monica Gandhi, San Francisco co-chair of the conference, which would have been held in that city and Oakland if the virus had not intervened. Instead, the gathering is unfolding online.

“Before covid, the theme of the meeting was: ‘We know how to treat, we know how to prevent, we know how to get to zero, we know how to do it. We need more political will. We need more commitment,’ ” Gandhi said.

The original theme of AIDS 2020, the huge international conference set to draw thousands of people from around the globe this week, was somewhat hopeful. There has been significant progress against HIV in some countries in recent years, and a continuing decline in new infections worldwide. One of the bright spots in reducing infections has been sub-Saharan Africa, even as the region lost 470,000 people to AIDS-related deaths in 2018.

Participants intended to highlight their determination to work harder for the poor, racial and ethnic minorities, and other marginalized people who have yet to share in those uneven advances.

The data shows “remarkable but highly unequal progress around the world,” Winifred Byanyima, executive director of UNAIDS, said at the session’s opening news conference Monday. “We have work to do.”

Then the coronavirus circled the globe and everything changed.

Even the biennial conference, which began Monday, has been radically transformed, from a global gathering to an entirely virtual proceeding — a technological leap of faith accomplished in less than four months. More than 20,000 people from 175 countries had signed up for four days of science, policy and activism and a separate one-day conference devoted to the coronavirus.

The meeting would have returned to the San Francisco Bay area 30 years after the tumultuous 1990 session that may be best remembered for the controversial 1987 rule, enacted by the federal government, that barred anyone with HIV from entering the United States. At the time, AIDS was almost always fatal, caused by a little-understood, much-feared virus. The lone medication worked poorly.

As a result of the ban, the United States did not host another international AIDS conference until the 2012 session in Washington, three years after President Barack Obama lifted the prohibition.

This year’s travel problem is a result of the coronavirus. Researchers who conducted an online survey of more than 13,000 lesbian, gay, bisexual, transgender and intersex people in 138 countries in April and May found that more than 91 percent were fully or partially locked down, leading to economic consequences for many.

Of the 1,140 among them who said they were HIV-positive, 26 percent said they had experienced “interrupted or restricted access” to the antiretroviral medication they take to treat the disease, and 55 percent of those said they had less than a month’s supply on hand.

Seventy-three percent of the full group said they could not meet their basic needs, and 1 percent had turned to sex work.

One of the most worrisome disruptions affects the distribution of “pre-exposure prophylaxis,” or PrEP, the medication that blocks transmission of HIV and has, along with treatment, transformed the infection from a death sentence to a manageable condition like diabetes or high blood pressure.

“In a lot of places, PrEP is nascent,” said Stefan Baral, an associate professor in the department of epidemiology at the Johns Hopkins Bloomberg School of Public Health and part of the team that conducted the survey. “The last few years has really witnessed a rapid expansion in PrEP programs.”

“Arguably, if people cannot access PrEP and testing and are just unable to do the things they need to do to keep themselves safe, we might see a slowing in the reduction in infections,” said Amrita Rao, a doctoral candidate in the same department who also worked on the survey, which will be presented at the conference.

At Ward 86, the safety-net clinic for people with HIV in San Francisco where Gandhi is medical director, the number of people whose viral loads were not suppressed rose 31 percent in April compared with a year earlier, she said.

In affluent Western nations, where access to treatment and prevention drugs is better, some men who have sex with men reported trouble accessing treatment, prevention and testing services in a small survey Baral and other researchers conducted in April.

“We do expect to see a significant impact on [HIV-related] morbidity and mortality from covid,” said Anton Pozniak, international chairman of the conference and president of the International AIDS Society.

“These are remarkable times — and defining times — for the global HIV movement and for the world,” he said at Monday’s news conference. “Every conversation we have now sits at the confluence of the covid-19 pandemic and a new global reckoning with systemic racism.”

In 2014, UNAIDS launched the 90-90-90 initiative, vowing that by 2020, 90 percent of HIV-positive people would be aware of that diagnosis, 90 percent of those with the infection would be receiving antiretroviral medication and the virus would be suppressed in 90 percent of people receiving treatment.

The conference will show that the world has not come close to meeting those goals. Fourteen countries have reached them, including tiny Swaziland, or eSwatini, where 27 percent of the population has HIV and public health efforts have reached 95 percent compliance in all three categories.

Still, 38 million people have HIV and 690,000 died of AIDS-related diseases in 2019, according to a report released Monday. About a third of the people with the infection were not receiving treatment.

About 1.7 million people were newly infected with the virus in 2019, despite striking progress in East and Southern Africa, where new infections have declined sharply in the past decade. The new battlegrounds are in Eastern Europe and Central Asia, where new HIV infections have risen by 72 percent since 2010, the report shows.

In the United States, where President Trump has vowed to halt transmission of HIV by 2030, HIV is increasingly a disease of minority men who have sex with men and intravenous drug users. It is concentrated in a small number of hot spots: the rural parts of seven mainly Southern states; the District; Puerto Rico; and 48 counties scattered across the country.

But progress here has stalled as well. The number of new infections each year has remained steady at between 36,000 and 38,000. About 1.2 million people have the virus, according to the Centers for Disease Control and Prevention.

And now the coronavirus hangs over it all.

“All we’ve done for the last 20 years is say, ‘Come into care’ . . . and then suddenly we tell people to stay away from the medical system,” Gandhi said.

She and other front-line workers have begun to spread the word that people with HIV or at risk of acquiring it are safe from the coronavirus in clinics and other health-care facilities — perhaps safer than in other places where it is spreading rapidly.

“We are going to make sure that we keep HIV as a major focus,” Pozniak said, “that we do everything we can so that prevention and testing and treatment carries on. But it is tough, with the resources moving away from HIV to covid. And you understand why that has to be.”