Asanda Zondi received week with orders to make her way to a health clinic in Vulindlela, South Africa, where she was participating in a research study that was testing a new device to prevent pregnancy and HIV. infection.

The trial was shutting down, a nurse told her. The device, a silicone ring inserted into her vagina, needed to be removed right away.

When Zondi, 22, arrived at the clinic, she learned why: The U.S. Agency for International Development, which funded the study, had withdrawn financial support and had issued a stop-work order to all organizations around the globe that receive its money. The abrupt move followed an executive order by President Donald Trump freezing all foreign aid for at least 90 days. Since then, the Trump administration has taken steps to dismantle the agency entirely.

Zondi’s trial is one of dozens that have been abruptly frozen, leaving people around the world with experimental drugs and medical products in their bodies, cut off from the researchers who were monitoring them, and generating waves of suspicion and fear.

The State Department, which now oversees USAID, replied to a request for comment by directing a reporter to USAID.gov, which no longer contains any information except that all permanent employees have been placed on administrative leave. Secretary of State Marco Rubio has said that the agency is wasteful and advances a liberal agenda that is counter to Trump’s foreign policy.

In interviews, scientists — who are forbidden by the terms of the stop-work order to speak with the news media — described agonizing choices: violate the stop-work orders and continue to care for trial volunteers, or leave them alone to face potential side effects and harm.

The United States is signatory to the Declaration of Helsinki that lays out ethical principles under which medical research must be conducted, requiring that researchers care for participants throughout a trial, and report the results of their findings to the communities where trials were conducted.

The New York Times identified more than 30 frozen studies that had volunteers already in the care of researchers, including trials of:

• Malaria treatment in children under age 5 in Mozambique

• Treatment for cholera in Bangladesh

• A screen-and-treat method for cervical cancer in Malawi

• Tuberculosis treatment for children and teenagers in Peru and South Africa

• Nutritional support for children in Ethiopia

• Early-childhood-development interventions in Cambodia

• Ways to support pregnant and breastfeeding women to reduce malnutrition in Jordan

• An mRNA vaccine technology for HIV in South Africa

It is difficult to know the total number of trials shut down, or how many people are affected, because the swift demolition of USAID in recent days has erased the public record. In addition to the disabled website, the agency no longer has a communications department. And the stop-work order prohibits any implementing agency from speaking publicly about what has happened.

Dr. Sharon Hillier, a professor of reproductive infectious diseases at the University of Pittsburgh, was until this week director of a five-year, $125 million trial funded by USAID to test the safety and efficacy of six new HIV prevention products. They included bimonthly injections, fast-dissolving vaginal inserts and vaginal rings.

Even trials that were not funded in whole or part by USAID have been thrown into turmoil because they were using medical or development infrastructure that was supported by the agency and is no longer operational. Millions of dollars of U.S. taxpayer funds already spent to start those trials will not be recouped.