Human-to-human transmission of the smallpox-related virus can occur via “respiratory droplets (and possibly short-range aerosols),” the WHO wrote in a June 4 situation update.

Large gatherings “may represent a conducive environment for the transmission of monkeypox virus if they entail close, prolonged, and frequent interactions among people, which in turn could expose them to contact with lesions, body fluids, respiratory droplets, and contaminated materials,” the agency added.

At a Wednesday press briefing Rosamund Lewis, WHO’s technical lead on monkeypox, said the risk of aerosol transmission is not fully known, but medical personnel caring for monkeypox patients should wear masks.

In terms of disease transmission, “droplets” and “aerosols” are different. Droplets are larger moisture particles that fall quickly to the ground, like drops of saliva expelled when a person coughs. Aerosols are much smaller particles that can linger in the air. If a virus is spread through aerosols, it is considered airborne.

Monkeypox is usually found in rural African areas where people have close contact with infected rats and squirrels. It is typically transmitted from human to human through close contact, which may include sex and could include contact with personal items like sheets and clothing. Airborne transmission is known to be possible but has yet to be confirmed.

But as global cases skyrocket—more than 1,000 cases have been reported in 27 countries in less than a month, the WHO said Wednesday—researchers are struggling to determine just how it’s being spread, and if the mode of transmission has changed.

On Monday the U.S. Centers for Disease Control raised its alert level for potential monkeypox transmission among travelers, advising them, among other things, to wear a mask while traveling. On Tuesday the masking advice disappeared from its website. When asked why, the CDC told Fortune it had removed the phrase “because it caused confusion.” The agency did not respond to a request for further elaboration, nor did it respond when asked if there was concern about airborne transmission.

While the CDC’s web page on monkeypox transmission for the general public does not mention potential airborne spread, its guidance for clinicians advises that tasks that could “resuspend dried material from lesions” such as dusting, sweeping, or vacuuming should be avoided while patients are hospitalized.

“Intubation and extubation, and any procedures likely to spread oral secretions, should be performed in an airborne infection isolation room,” CDC adds.

New monkeypox genetic variants found in U.S.

There were 40 U.S. cases as of Wednesday, according to CDC data. No fatalities have been reported thus far in the U.S. or globally, and health officials have noted that all tested specimens have belonged to the West African clade, the milder of two clades, with a death rate typically around 3%.

New mutations within the clade have been found, however. “At least two genetically distinct variants” of monkeypox have been discovered in the U.S., CDC officials said on a Friday press call.

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., they said, adding that not all U.S. cases identified have been genetically sequenced.

Of the 10 U.S. cases sequenced by the CDC as of Friday, “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.

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.

1,400 additional cases in Africa this year so far

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 indicative of a “less than complete picture of the long-standing 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 on Friday, saying that the multiple new 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.”

WHO director general Tedros Adhanom Ghebreyesus noted at a Wednesday press briefing that 1,400 suspected cases of monkeypox have been identified in Africa so far this year, with 66 deaths.

“This virus has been circulating and killing in Africa for decades,” he said. “It’s an unfortunate reflection of the world we live in that the international community is only now paying attention to monkeypox because it has appeared in high-income countries.”