From the start of the global monkeypox outbreak, researchers have hoped that people could only spread the virus once they developed symptoms.
In addition to painful lesions, monkeypox symptoms can include fever, chills, swollen lymph nodes, headaches, muscle aches, back pain and fatigue.
The study is the first to demonstrate that poxviruses, which include both monkeypox and smallpox, can spread in this manner.
Past research suggested that asymptomatic and pre-symptomatic transmission of monkeypox was possible, but only symptomatic transmission had been documented.
With this new observation about pre-symptomatic spread, Freeman said, it may be impossible to reach a point of zero cases.