READ THIS NEXT: This Easy-to-Miss Symptom Could Be Your First Monkeypox Sign, CDC Warns. The international health community is suddenly worried about monkeypox because it appears to be spreading more rapidly than at any point since scientists first discovered it in a colony of lab monkeys in 1958. According to the Centers for Disease Control and Prevention (CDC), the virus has typically passed from an infected animal to humans and rarely from person to person. But the most recent surge in dozens of countries outside of where the virus is ordinarily endemic may mean that “widespread human-to-human transmission is currently underway,” Maria van Kerkhove, MD, an infectious disease expert with the World Health Organization (WHO), said in a statement on May 29. Usually, the agency says that the virus causes symptoms similar to smallpox that are milder in comparison and usually begins with fever, headache, muscle aches, backaches, chills, and exhaustion. Notably, one key difference between the viruses is that monkeypox usually causes swelling in the lymph nodes while smallpox does not. Patients then typically begin to develop a painful rash one to three days after their fever first appears, starting with flat lesions that then raise as they fill with pus before eventually falling off. The CDC says that the incubation period from infection to the first signs of sickness usually ranges from seven to 14 days but can be as long as five to 21 days. And while the WHO notes that monkeypox can be fatal in one to 11 percent of infected patients, cases in the latest outbreak all appear to be caused by the less virulent strain of the virus, The Washington Post reports.ae0fcc31ae342fd3a1346ebb1f342fcb However, while health officials have been urging anyone who develops monkeypox symptoms to seek medical attention immediately, the latest outbreak is proving more challenging to spot in some cases. That’s partly because one telltale sign of the virus may not be as prominent as usual, NPR reports. According to Donald Vinh, MD, an infectious disease doctor at McGill University Health Centre in Montreal, some patients are experiencing more minor, less pronounced skin rashes—including one diagnosed case that had just one small lesion. “The patient’s skin lesion that he sampled to confirm the diagnosis is extremely subtle. It’s not what you’re seeing on the Google pictures of monkeypox,” he told NPR.

RELATED: For more up-to-date information, sign up for our daily newsletter. Vinh points out that the typical course of the virus involves the two phases, with the first bringing initial flu-like symptoms and fever before the skin rash develops in phase two. But in several recent cases, it’s been found that patients aren’t developing rashes on their face or extremities as normal before spreading, but rather seeing lesions remain isolated in one area without advancing, NPR reports. “You don’t have head-to-toe skin pox lesions,” Vinh told NPR. “Instead, it’s localized to just one region of the body, like the genital regions. And some people have just one or two pox. So it’s not numerous,” adding that “sometimes it’s not even a pox, but rather an ulcer or a crater.” And besides the telltale rash, Vinh reports that he and other doctors have observed patients who never develop the initial fever and flu-like symptoms. In other cases, they’ve followed the development of a skin rash. Others have reported only experiencing one swollen lymph node if any at all, NPR reports. While the virus’s rapid spread has caused alarm and raised a series of questions, no deaths have yet been reported. In the U.K., which has reported the most cases during the outbreak so far, health officials remained optimistic in an update issued on May 31, writing: “The risk to the U.K. population remains low, but we are asking people to be alert to any new rashes or lesions, which would appear like spots, ulcers or blisters, on any part of their body.” But due to the apparently evolving nature of the virus, some officials in the U.S. began to question whether or not the virus may have been spreading domestically for longer than previously realized after a CDC report found that two genetically distinct monkeypox variants were circulating stateside, CNN reported. As a result, the agency urges the public and health care providers to “stay vigilant” to help slow the spread of the virus. “I want to emphasize that this could be happening in other parts of the United States. There could be community-level transmission that is happening, and that’s why we want to really increase our surveillance efforts,” Jennifer McQuiston, MD, deputy director of the CDC’s Division of High Consequence Pathogens and Pathology, told CNN. “We want to really encourage physicians that if they see a rash and they’re concerned it might be monkeypox, to go ahead and test for that.” READ THIS NEXT: If You Notice a Blister Here, Get a Blood Test, Experts Say.