Monkeypox — the oddly named infectious rash that was unfamiliar to most Americans until last month — continues to spread very slowly in Pennsylvania, New Jersey, and most of the United States.

As far as anyone knows, that is.

Infectious-disease experts say the official CDC total — 173 cases as of Thursday afternoon, including two in Philadelphia and one in New Jersey — is almost certainly an undercount.

The reasons behind that statement require a bit of explanation, but the end result is a push for expanded testing to identify any cases that may have been missed.

The U.S. Department of Health and Human Services started shipping test kits to five commercial laboratory companies this week, boosting capacity by thousands of tests per week, the agency said Wednesday. And others have called for allowing academic medical centers to conduct tests, as they did in the early days of the COVID-19 pandemic, when capacity shortages at state labs meant some patients waited up to a week for their results.

A failure to test more people for monkeypox will hamper efforts to keep the emerging threat under control, said epidemiologist Michael LeVasseur, an assistant professor at Drexel University’s Dornsife School of Public Health.

“When it comes to our public health response, it is the exact same as March 2020,” he said. “We are in an emergency.”

The good news: The variety of monkeypox circulating in the United States causes mild to moderate disease, and no one here has died. Still, the telltale skin lesions are often painful, and they can result in lasting scars.

The concern is sufficient that on Thursday, a World Health Organization committee met to discuss labeling the outbreak as a global health emergency — though the disease has been common in parts of Africa for many years.

Here is the latest on how doctors decide whom to test, and the emerging science on whether monkeypox, which spreads primarily through skin contact, may also be a sexually transmitted disease:

Why are some monkeypox cases being missed?

Many of the people who’ve tested positive for monkeypox have no idea how they were exposed.

Almost by definition, that means there are more cases than what’s been officially reported to the CDC, Brown University epidemiologist Jennifer B. Nuzzo and Cornell University physician Jay K. Varma wrote this week in the Washington Post.

“If a patient doesn’t know who infected them, that means there is at least one person in the community who is spreading infection to others without knowing it,” they said.

The answer, they say, is more testing.

Yet the solution is not just expanding capacity but also teaching physicians and patients to be on the lookout.

In past outbreaks in Africa, patients developed more than 100 lesions on their bodies, whereas some U.S. patients have just one or two, for reasons that aren’t yet clear, LeVasseur said.

As a result, physicians who have never encountered monkeypox may fail to recognize it, said Christina Wojewoda, chair of the microbiology committee for the College of American Pathologists.

“It seems to be a much more subtle presentation,” she said. “It might be mistaken for something like herpes or varicella zoster virus [chicken pox] or syphilis,” she said.

What are the signs of monkeypox?

The CDC recommends testing anyone with a “characteristic rash” — typically, lesions that start out as flat circles and grow into protrusions shaped like pencil erasers, eventually filling with pus.

Testing may also be recommended for individuals with no lesions if they were exposed to an infected person and have other symptoms such as fever and swollen lymph nodes, said Donna Leusner, a spokesperson for the New Jersey Health Department.

“If an individual has atypical or even absent rash, but the clinical picture and epidemiology is strong, testing would be approved,” she said.

That additional evidence is key, infectious-disease experts say.

For example, many cases in the current outbreak have been identified in men who have sex with men, so that type of patient history could be a reason to test someone with possible symptoms. That’s why the added testing this week is essential, as it is the last week of Pride Month — which features large gatherings where the virus could be more likely to spread, LeVasseur said.

But if a 5-year-old girl has no known exposure to a monkeypox patient, yet develops a rash and was not vaccinated against chicken pox, testing would not be warranted, said Wojewoda, who directs the microbiology lab at the University of Vermont Medical Center.

“We don’t want to just test everybody who has a rash,” she said.

Do we have enough monkeypox test kits?

For now, testing patient samples for monkeypox is a two-step process.

First, a sample is tested at a state or regional lab to see if it contains DNA from a broader category of viruses called orthopox viruses.

The state Health Departments in Pennsylvania and New Jersey both say they’ve had plenty of capacity to conduct these first-round tests so far, but did not provide specifics. New Jersey officials say their lab has been conducting between five and 10 of the tests per week, with just one positive result so far.

Collectively, the state and regional labs are able to perform more than 8,000 orthopox tests per week. The new test kits for commercial labs will expand that capacity to tens of thousands of tests per week, the Biden administration told the Washington Post.

If a sample comes back positive for an orthopox virus, it is presumed to be monkeypox, as that is the only orthopox virus in wide circulation. But just to be sure, such samples are sent on to the CDC for confirmatory testing.

Vaccines are available and may be recommended for those who are exposed to an infected person.

Is monkeypox sexually transmitted?

Researchers have not yet determined whether monkeypox is sexually transmitted.

Monkeypox spreads primarily through skin-to-skin contact. But when the disease began its unusual spread outside of Africa this spring, epidemiologists determined that many of the cases occurred in men who have sex with men.

That could mean it was spreading via skin contact during sex. It’s also possible that the illness can be transmitted through bodily fluids, as viral DNA has been detected in the semen of several patients in Europe.

The distinction is important, said David Cennimo, an infectious-diseases specialist at Rutgers New Jersey Medical School.

If the illness could spread via semen, when no lesions or other symptoms are present, then people having sex would be wise to take precautions, he said. That could include using condoms — though it’s not clear that condoms would prevent all transmission, he said.

It’s also possible that the disease has been seen more often in men who have sex with men because they, and their physicians, are especially attuned to signs of sexually transmitted infection due to the HIV epidemic, said Cennimo, an associate professor of medicine and pediatrics.

“You have to pause and think to yourself, is it disproportionately happening among men who have sex with men?” he asked. “Or is it that they’re the ones who are getting care?”