Asanda Zondi received a startling phone call last Thursday, 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 H.IV. 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 Ms. 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22, arrived at the clinic, she learned why: The U.S. Agency for International Development, which funded the study, had withdrawn financial support and had issued a stop-work order to all organizations around the globe that receive its money.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 Trump freezing all foreign aid for at least 90 days. Since then, the Trump administration has taken steps to dismantle the agency entirely.

Ms. 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 U.S.A.I.D., 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 President 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.