“I’ve never been that sick in my life,“ said Mr. Feltner.

More than two months after monkeypox was detected in the U.S. as part of a global outbreak mostly among men who have sex with men, transmission shows no sign of slowing. The U.S. has surpassed Spain as the country with the most known cases. Federal officials are considering whether to declare monkeypox a public-health emergency.

Patients said they are navigating the outbreak without sufficient guidance from physicians and public-health officials on how to treat the disease and prevent its spread. Mr. Feltner said he wasn’t able to get vaccinated before travel because Iowa gave priority to people who had been exposed to monkeypox. He said he was mocked by healthcare providers when he attempted to seek care and struggled to find adequate expert information about the disease. He said he has relied on social media and the anecdotal accounts of other patients to fill the gaps.

“Trying to get questions answered was impossible,” said Mr. Feltner.

Public-health experts and community advocates said the Biden administration and the Centers for Disease Control and Prevention were too slow at responding to the outbreak when it first emerged and continue to take insufficient action to stem the spread of the virus.

“We’re not getting streamlined communication to communities. People are making things up as they go. That’s not the best public-health strategy,” said Jason Rosenberg, a member of ACT UP NY, an HIV/AIDS advocacy organization.

The CDC said it was wrong to suggest that it hadn’t taken early and adequate action to mitigate the outbreak and to educate communities about monkeypox.

“All summer, [the] CDC has been working with partners to help put information in the hands of people who may be at highest risk for contracting monkeypox,” an agency spokesperson said, adding that the CDC continues to work with community health organizations to raise awareness about the virus.

The Department of Health and Human Services didn’t immediately respond to requests for comment.

To date, there have been more than 5,800 confirmed or suspected cases in the U.S., mostly among men who have sex with men, federal health officials said. Epidemiologists said the virus is exploiting close-knit social and sexual networks, but knowledge of how the virus is spreading in this current outbreak remains incomplete.

“We have to be very humble about what we know and what we don’t,” said Anne Rimoin, an epidemiologist at the University of California, Los Angeles, who spent two decades studying monkeypox in Africa, where the disease has long been endemic. “What we know is based on studies done in very different epidemiological and ecological contexts. We need to know much more about transmissibility.”

The World Health Organization, which declared monkeypox a global health emergency, says monkeypox is most commonly spread through close contact with an infected person’s rash, lesions and bodily fluids. The virus can also spread via fabrics and other materials, and through prolonged exposure to an infected person’s saliva or mucus.

Aerosol, also known as airborne, transmission of the virus hasn’t been considered to be of significant concern by public-health experts, but some early research suggests it could be possible in certain conditions. Airborne transmission is defined as the dissemination of a virus through very small droplets that remain suspended in the air over long distances and time.

Public-health experts recommended that people at risk for infection get vaccinated when possible, not share bedding or towels, wash hands often, and avoid intimate physical contact with someone with rashes or sores, or limit sexual partners altogether.

A person can spread monkeypox until all their lesions have scabbed and the scabs have fallen off, infectious-disease experts said. Public-health officials have advised potentially infectious people to remain quarantined for the duration of their illness. Monkeypox symptoms typically last two to four weeks, the CDC said.

Most monkeypox cases in the U.S. have been mild, though moderate and severe cases have been reported. A recent study in the New England Journal of Medicine of more than 500 monkeypox patients in 16 countries found that 13% were hospitalized, mostly for pain.

It is hard to predict who will experience a severe case, said Peter Chin-Hong, a professor of medicine at the University of California, San Francisco, noting that patients in his care haven’t been immunocompromised or very old. “The people who are hospitalized are very, very sick,” he said.

Josh Watson of Chicago said he developed lesions on different parts of his body, including some in his throat that made it difficult to eat and drink. He was hospitalized and treated with the antiviral drug tecovirimat in mid-July. The drug, sold by New York-based

Siga Technologies Inc.

under the brand name TPOXX, has been approved by the Food and Drug Administration for the treatment of smallpox, a disease closely related to monkeypox.

Amid the outbreak, the FDA and CDC have approved expanded access of TPOXX to monkeypox patients, but healthcare providers said obtaining the drug has been burdensome. Until recently, it took providers several hours to complete extensive FDA and CDC paperwork and other requirements to get the drug to patients, said Cathy Creticos, medical director of infectious disease at Howard Brown Health, a nonprofit LGBTQ healthcare and social services provider in Chicago. The CDC simplified the protocol to obtain TPOXX late last month.

Twenty days after his first monkeypox symptoms appeared, Mr. Watson said he remained quarantined with a lesion on his foot and lingering fatigue.

“The risks of monkeypox have been severely downplayed,” Mr. Watson said. “People need to know that the risk is greater, the symptoms are greater. What I’d heard is that you get a rash for a couple of weeks and then it’ll go away, you won’t die. But no one was talking about the pain, the difficulty urinating or having bowel movements.”

Some patients with moderate to severe symptoms said their symptoms improved significantly after taking TPOXX. Healthcare providers said most monkeypox cases in the U.S. have resolved without medical intervention.

Mr. Feltner said it took many days before he was able to get access to testing and treatment for monkeypox.

He said a nursing assistant at an urgent-care clinic in Iowa City laughed at him when he suggested he had monkeypox. A doctor at the clinic diagnosed him with herpes and refused to test him for monkeypox, Mr. Feltner said. The doctor sent him home with Valtrex, an antiviral drug used to treat herpes.

The medication didn’t work. “Things just got worse,” said Mr. Feltner, who was eventually hospitalized and given TPOXX.

“Our healthcare system here is definitely not ready for this disease,” Mr. Feltner said.