By Sandra Rose  | 

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The CDC first detected cluster cases of monkeypox virus in the United States in 2003.

As cases of a "new" monkeypox spreads in US cities, experts are questioning whether the virus is sexually transmitted.

Symptoms of monkeypox include fever, headache, muscle aches, weakness, and swollen lymph nodes (in neck, armpits and groin areas. Symptoms are followed by blisters that burst and crust over.

Health officials say monkeypox infection can be prevented by vaccines stockpiled from previous outbreaks.

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The viral disease is believed to be spread to humans by domestic prairie dogs and other exotic animals being kept as household pets.

Health officials wonder if the recent spike in monkeypox cases in the U.S. and Europe is caused by a "new" virus.

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Through the first months of 2020, more than 4,500 cases and 171 deaths have been tracked by the World Health Organization (WHO).

Now officials fear that monkeypox has mutated into a new, more contagious virus that is sexually transmitted.

"The cases currently being detected are among those engaging in sexual activity, and the symptoms are unfamiliar to many," the WHO said in a statement.

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Dr. Jeremy Faust discovered that all 89 confirmed monkeypox cases in Europe are among men who had sex with men.

Of the cases confirmed in Europe, Dr. Faust said, "all 89 are males, a large number of whom are young and middle-aged adults."

Dr. Faust said the evidence "suggests strongly" that monkeypox may still be primarily spread through sexual contact, a known major mode of transmission.

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He said if the virus was spread through the air via respiratory droplets or surface to skin contact, some women would be infected by now.

Virologist Angela Rasmussen believes the virus is spread within a sexual network of men who then travel, which explains why the virus is popping up in other countries.