By the time Sydney Towle graduated from Dartmouth College in 2022, she had a growing social media side hustle.

On TikTok, where she posted videos, her fans watched her perform dance moves in her kitchen and lip-sync to popular songs. She modeled clothing and posed in bikinis on the beach. She gallivanted around Europe with friends.

Within a year, she was an influencer in full, with more than 450,000 followers.

But her content took a sharp turn in August 2023. In a pink bikini top, her face stained with tears, she spoke directly to the camera. “I have cancer,” she said. “I am strong, so I’ll be good.” She flashed a thumbs-up and an awkward smile.

Her diagnosis, she said, was cholangiocarcinoma, a rare and aggressive cancer of the bile ducts.

Ms. Towle’s social media posts grew more frequent and personal, as she joined the ranks of influencers in what is known as CancerTok. She made videos of herself exploring New York City, where she said she had moved to be close to Memorial Sloan Kettering Cancer Center; struggling with exhaustion and nausea she attributed to chemotherapy; and crying in grief for the carefree life she no longer led. And her growing online fan base — before long, she had more than 760,000 followers — routinely cheered her on:

“We are all rooting hard for you girl!!!” one person posted on TikTok. “Keep up the good fight.”

“Your positive outlook through all of this inspires me daily,” another said.

But on Reddit, in a more skeptical and caustic corner of the internet, an army of angry critics was assembling.

“I literally feel like punching her in the face,” wrote one commenter.

Ms. Towle’s antagonists had quickly concluded that she was leveraging her story of illness for public sympathy and financial benefit. They scrutinized her every word and image, building a case that she was misleading the public.