Moments before posting her first food review on social media in May, Christina Hart was sitting in her red pickup truck, in a carwash parking lot, and “crying out to God,” she said.

“That was my breaking point,” said Ms. Hart, 29, of the Nashville area, who had recently lost her job and has two young children, including a daughter with severe autism whose care can be “all-consuming,” she said.

To raise her spirits, she filmed herself rating two items from a coffee shop in Hendersonville, Tenn.: a sausage-biscuit sandwich (six out of 10; “really dry,” she said) and a matcha lemonade (10 out of 10). Overnight, the video racked up thousands of views.

Since then, Ms. Hart has reviewed Cuban sandwiches, coffee, cookies and more around the Nashville suburbs and has gained about 20,000 followers across social media platforms, she said. She has used her videos to raise money for a struggling local business.

“It’s pretty much gave me hope,” she said through tears. “For having a way to take care of my kids.”

But this July, her fledgling career stopped as fast as it started. Ms. Hart became one of thousands around the country debilitated by cyclosporiasis, an intestinal illness caused by a microscopic parasite hidden among heads of lettuce, stalks of broccoli, crates of berries and more.

Suddenly, she could not leave her house, she said. “I’m so sorry,” she told restaurant and coffee shop owners. “I can’t get off the toilet.”

On July 14, in lieu of her weekly food reviews, she posted a selfie from bed on Facebook. “Well,” she told her followers after a doctor’s appointment, “#Cyclospora has officially entered the chat.”

The cyclospora parasite, spread through food or water contaminated with human feces, infects the intestines and can cause cramping, diarrhea and fatigue. Though infections are common, the U.S. is currently in the midst of its largest-ever breakout, with more than 18,000 cases confirmed or under investigation across 45 states.

Many have been linked to shredded lettuce that Taylor Farms supplied to Taco Bell, but the parasite also clings to fresh produce. Raspberries — which Ms. Hart said she often ate in the morning — are a common carrier.

“I wasn’t ever in a million years thinking I had a darn parasite,” said Ms. Hart, who initially assumed she had food poisoning and was unsure how she got the parasite. Tracing cyclosporiasis is difficult because it can incubate inside someone for up to 14 days before producing symptoms.

Ms. Hart said she lost 17 pounds in less than four weeks.

While that’s a high figure, “a significant illness can cause significant weight loss,” said Dr. Stephen Feagins, medical director of the public health department of Hamilton County, Ohio, where several cases of the parasite have been reported. Much of that weight could be fluids, he added.

Eventually, Ms. Hart reached a “state of delirium,” she said. How could she be a food reviewer without eating any food?

“I’ve done so many things in my life that have failed,” Ms. Hart said. “It just makes you question: Is this what I’m supposed to do? Am I imagining it? Was this a sign that I’m supposed to stop?”

But posting about the illness brought her videos more attention than ever. People sent messages of support, encouraging her to keep uploading reviews. Others shared their own experiences with the parasite.

“This whole cyclospora thing — I thought it was actually a curse, but it ended up being a blessing,” she said, adding that she was upgrading her video equipment and hoped to one day expand her reviews beyond Tennessee.

“If poop is what lands me there,” she said, “I guess that’s what lands me there.”