Both cases share common ancestors with strains that have developed in Nigeria since 2017 and are similar to those seen in a 2021 case imported to the U.S., officials said, adding that not all of the 20 U.S. cases identified as of Friday have been genetically sequenced yet.

Of the 10 U.S. cases sequenced by the CDC so far, “most are closely related to the cases in Europe, but three are unrelated and represent separate importations from Africa or the Middle East,” Eric Toner, senior scientist and health security expert with the Johns Hopkins Bloomberg School of Public Health, told Fortune on Friday.

The news of the newly identified strains comes amid reports of transmission patterns atypical for the disease. More than 800 cases have been identified globally since last month, but such extensive spread is not normal for the virus. The pathogen is usually found in rural African areas where people have close contact with infected rats and squirrels, and is typically transmitted from human to human through close contact or large respiratory particles. It’s usually difficult to catch, with household transmission rates around 10% or lower.

Symptoms also appear to differ from classic cases—at least in some instances—with recent reports of lesions more subtle than usual and some cases involving just one lesion, U.S. health officials acknowledged Friday.

But no current information suggests a recent mutation that would lead to a change in symptoms or mode of spread, Toner said. Instead, newly noted differences are likely reflective of a “less than complete picture of the longstanding spectrum of monkeypox illness in West Africa.”

Mild cases of monkeypox like some recently reported in the U.S. are probably “not new, but may not have been easily recognized in Africa,” Toner said. “People covered in pox are easy to recognize and are likely to seek medical care. A mild case with a solitary lesion is easy to miss or misdiagnose.”

Nathan Grubaugh, an associate professor of epidemiology at the Yale School of Public Health, agreed with Toner, saying that the multiple strains of monkeypox recently reported likely evolved and have been circulating in Africa, where transmission isn’t well tracked.

“The big problem is there is a lot of ongoing transmission in endemic areas that there aren’t great surveillance systems for,” he said. “We as a Western society aren’t paying a lot of attention to what is happening there. We’re just missing a lot of information.”

The new strains of monkeypox still belong to the West African clade, Grubaugh said, as do all other recently reported cases. The clade is typically less severe than the other known clade, Congo Basin.

The majority of cases continue to be identified among men who have sex with men, U.S. health officials said Friday. Experts have recently questioned whether the virus is now being sexually transmitted but acknowledge that it’s difficult to separate sexual transmission from close-contact transmission.

The threat to the public remains low, officials said Friday. But “community-level transmission” could be happening in the U.S., one official said, adding that it’s “certainly possible” monkeypox had been circulating in the U.S. before the first case was identified in Massachusetts on May 18, “but not to any great degree.”

Transmission of monkeypox from West Africa and Nigeria may have been occurring undetected over the last several months, Dr. Albert Icksang Ko, a professor with the Yale School of Public Health, told Fortune on Friday.

Patients identified so far have “travel histories from many different regions, including the Middle East”—a worrisome sign that multiple chains of transmission are occurring. Icksang Ko thinks it’s “plausible” that introductions have been happening since a 2017 resurgence in Nigeria, “and they just went under the radar.”

While many recent cases have so far been linked to men who have sex with men, Icksang Ko cautioned that “many times early in an outbreak, you’re only seeing the tip of the iceberg in specific risk groups.”

“This is not just a gay problem,” he said, emphasizing that the virus is known to be transmitted via close contact, which can include sex but isn’t limited to it.

On the bright side, recent monkeypox cases seem to be less severe than usual, with no fatalities reported in the U.S. or globally despite a reported fatality rate of 10% in Africa.

But scientists still must determine “if we are having widespread community transmission, and if so, what we need do about it,” he said.

A milder version of smallpox

Monkeypox, a rare disease related to smallpox and cowpox, was first identified in 1958 among colonies of monkeys kept for research, according to the CDC. African rodents and nonhuman primates like monkeys may harbor the disease and infect people. The first human case was recorded in 1970, in the Democratic Republic of Congo, where the majority of infections now occur. It has since been reported in humans in other African countries like Cameroon, Central African Republic, and Ivory Coast.

Prior to this year’s outbreak in Europe, cases had been reported in the U.S. as well as Israel, Singapore, and the United Kingdom, according to the CDC. Last year two cases were identified in the U.S., one in November and one in July. Both involved travelers who had just returned from Nigeria. Human monkeypox was first reported outside of Africa in the U.S. in 2003, when 47 confirmed and probable cases were reported. All cases had contact with pet prairie dogs that had been housed near small mammals—including rope squirrels, tree squirrels, and African giant-pouched rats—imported from Ghana.

Symptoms are similar to, but milder than, that of smallpox, according to the CDC. Initial symptoms usually include fever, headache, muscle aches, and exhaustion. Within one to three days, patients develop a rash, usually starting on the face and then spreading to other parts of the body. Lesions progress through various stages before scabbing. The illness usually lasts two to four weeks. The typical incubation period is 7 to 14 days but can range from 5 to 21 days.