Struggling to contain a raging measles epidemic in West Texas, public health officials increasingly worry that residents are relying on unproven remedies endorsed by Robert F. Kennedy Jr., the health secretary, and postponing doctor visits until the illness has worsened.

Hospitals and officials sounded an alarm this week, issuing a notice explaining which measles symptoms warranted immediate medical attention and stressing the importance of timely treatment.

“I’m worried we have kids and parents that are taking all of these other medications and then delaying care,” said Katherine Wells, director of public health in Lubbock, Texas, where many of the sickest children in this outbreak have been hospitalized.

Some seriously ill children had been given alternative remedies like cod liver oil, she added. “If they’re so, so sick and have low oxygen levels, they should have been in the hospital a day or two earlier,” she said.

The growing outbreak has spread to nearly 260 people in Texas. So far, 34 patients have been hospitalized, and one child has died. In neighboring New Mexico counties, the virus has sickened 35 and hospitalized two. Two cases in Oklahoma have also been linked to the outbreak.

Texas health officials believe the true number of cases is far higher. In all, there have been 301 measles cases in the United States this year, the highest number since 2019, the Centers for Disease Control and Prevention reported on Friday.

In his first public statements about the outbreak, Mr. Kennedy faced intense backlash for minimizing the situation, saying it was “not unusual” and falsely claiming that many people hospitalized were there “mainly for quarantine.”

In the following weeks, Mr. Kennedy altered his approach, offering a muted recommendation of vaccines for people in West Texas while also promoting unproven treatments like cod liver oil, which has vitamin A, and touting “almost miraculous and instantaneous” recoveries with steroids or antibiotics.

There is no such cure for measles, only medications to help manage the symptoms. Vaccination is the most effective way to prevent the infection.

While doctors will sometimes administer high doses of vitamin A in a hospital to help manage severe cases of measles, there is no credible evidence that supplements are effective for treating or preventing measles.

Experts also noted that antibiotics, which fight bacterial infections, may be used to treat secondary infections but do not stop measles itself, which is a virus.

In Gaines County, Texas, the epicenter of the measles outbreak, alternative medicine has always been popular. Many in the area’s large Mennonite community, where most of the measles cases have been clustered, avoid interacting with the medical system and hold to a long tradition of natural remedies.

In the last few weeks, drugstores in West Texas have struggled to keep bottles of vitamin A pills and cod liver oil supplements on their shelves.

And this week, doctors at Seminole Memorial Hospital, which sits at the center of Gaines County, noticed that the number of patients coming in for measles symptoms suddenly dropped. Those who did show up were sicker than patients seen in previous weeks.

Even while cases in the community increased, Dr. Leila Myrick, a physician at the hospital, said she performed half the number of measles tests, compared with those the week before.

She worried that her patients were instead going less than a mile away from the hospital to a pop-up clinic, where a doctor from a neighboring city had been doling out alternative remedies, like cod liver oil and vitamin C.

The physician, Dr. Ben Edwards, is well known in the area for producing podcasts that often discuss the dangers of vaccines, and for his wellness clinic in Lubbock, which rejects central tenets of medicine, like the idea that germs cause certain diseases.

In an interview with Fox News, Mr. Kennedy said he had spoken with Dr. Edwards (whom he mistakenly called Dr. Ed Benjamin) and learned “what is working on the ground.”

In an email relayed through an employee, Dr. Edwards confirmed that he had talked to Mr. Kennedy for about 15 minutes in what he described as an “information gathering” phone call. Dr. Edwards declined to speak directly with The New York Times.

In the following days, hundreds of people from the Mennonite community lined up at Dr. Edwards’s makeshift clinic, held behind a local health food store, said Tina Siemens, who helped organize the event.

Mrs. Siemens said people seeking treatment for active measles infections and those who hoped to prevent one were in attendance.

To get enough supplements for the clinic, Dr. Edwards had enlisted one of his patients, a pilot, to fly to Scottsdale, Ariz., and pick up nearly a thousand bottles of vitamin C supplements and cod liver oil, both as a lemon-flavored drink and unflavored soft gels, said an owner of the supplement company, Patrick Sullivan.

“How much do you have in stock, and how quickly could you get it to me?” Mr. Sullivan recalled Dr. Edwards asking.

The treatments were free, Mrs. Siemens said. Members of Children’s Health Defense, an anti-vaccine nonprofit that Mr. Kennedy helped found before becoming health secretary, created a donation page online that has raised more than $16,000 to help cover the cost of “essential vitamins, supplements and medicines.”

Measles symptoms often resolve on their own within a few weeks. But in rare cases, the virus can cause pneumonia, making it difficult for patients, especially children, to get oxygen into their lungs. There could also be brain swelling, which can cause lasting problems, like blindness, deafness and intellectual disabilities. Both complications can be deadly.

During this outbreak, hospitalized children with pneumonia have had to be intubated, Ms. Wells, the Lubbock health director, said. In those circumstances, timely care can mean the difference between life and death.

Unproven remedies have for decades made measles outbreaks more deadly, said Patsy Stinchfield, immediate past president of the National Foundation for Infectious Diseases.

She worked as a nurse practitioner at a hospital in Minnesota during a measles outbreak in 1989 that killed several children. Two of them arrived at her hospital in critical condition after their parents had tended to them at home with traditional healing therapies.

“They keep their child at home too long, and they try these home remedies,” she said. “They went straight from the E.R. into the intensive care unit and they died.”