As the number of monkeypox cases continues to rise in several countries, scientists believe that the disease may have been spreading undetected for weeks, months, or even years.

Before the first cases outside of Africa, it is likely that the skin marks associated with monkeypox were being confused with other sexually transmitted infections leading to a possible delay in diagnosis, Amesh A. Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told NBC News.

So far, 780 confirmed cases of monkeypox have been reported since the outbreak began on May 13 this year, with at least 27 World Health Organization (WHO) member states reporting cases, including the US and Canada.

Also Read | How the White House has responded to monkeypox outbreak in US

Investigations are underway to find more details about the monkeypox which has spread through countries. It was earlier known to be found in African countries such as the Democratic Republic of the Congo, Cameroon, and the Central African Republic.

The World Health Organization said the “sudden and unexpected” spread of monkeypox suggested that “there might have been undetected transmission for some unknown duration of time followed by recent amplifier events.”

Adalja added: “What’s likely happened is an endemic infectious disease from Africa found its way into a social and sexual network and then was greatly aided by major amplification events, like raves in Belgium, to disseminate around the world.”

Scientists are also trying to understand whether the virus has seen some genetic change that has enabled it to spread outside of Africa. 

Monkeypox is a disease caused by a virus of the same name. It was first found in monkeys kept for animal testing in 1958. The first human case was detected in 1970. The symptoms are similar to smallpox, but are much milder.

Also Read | Officials detect 2 monkeypox strains in the United States

The incubation period—the interval between infection and symptoms—is usually from six to 13 days.

Symptoms usually start with fever, headache, other body aches, swelling of the lymph nodes and lack of energy, according to the WHO. Within a day or a few days, a skin rash appears that can affect various parts of the body. Symptoms usually last from two to four weeks.