Again and again, it served her videos attributing Zika to bad vaccines or international conspiracies. Other mothers had the same experience, and they shared their findings on group text messages.

Some of the YouTube videos had been staged to resemble news reports or advice from health workers. The recommendation systems are still promoting them, the Harvard analysis found, recommending them alongside more reputable medical advice and surfacing them as top search results.

A spokesman for YouTube confirmed the findings, calling the results unintended, and said the company would change how its search tool surfaced videos related to Zika.

Ms. Oliveira dos Santos knew that the internet could be unreliable. And she believed in vaccines: She knew that they could protect children from serious diseases. But after watching the videos, she felt paralyzed by doubt.

Though she gave her child standard childhood vaccines, she said, “I was scared to give any more vaccines to my daughter after that.” She and her mother have both stopped taking vaccines as well.

That wasn’t the only issue on which YouTube altered her thinking.

She had not initially been a supporter of Mr. Bolsonaro, she said. But friends of hers kept sending her videos about him. So she turned to YouTube to find out more.

“I searched, and I became convinced by what he said, and what he would change and what he would improve,” she said. “That influenced me a lot.”

In October, she voted for him.