(NewsNation) — A fifth child in the United States has tested positive for monkeypox, furthering the spread of a disease that’s left public health experts frustrated.

So far, two children each in Indiana and California were diagnosed, as well as a child visiting Washington, D.C., from out of the country.

As the world tries to put a lid on the growing outbreak, new data from Africa suggests children younger than 8 may be especially at risk for more severe disease. Doctors are still trying to figure out why.

“I think it has to do with the maturity of the immune system. We don’t have a lot of details about it,” said Dr. Peter Hotez, a pediatrics professor at the Baylor College of Medicine, during an appearance Wednesday on “NewsNation Prime.”

Hotez believes the five children in America likely picked up the disease from someone they live with. The disease spreads through respiratory droplets, skin to skin contact, and oral to skin contact.

That list of relatively intimate sources of transmission has led some to write it off as a sexually transmitted disease. Though the Centers for Disease Control and Prevention has said sexual contact is a main source of transmission, Hotez believes outlier cases become more likely with continued spread.

“We should expect spillover from the current high risk group to the general population if we allow those cases to continue to grow,” Hotez said.

Hotez is also frustrated at the seemingly preventable nature of this outbreak.

“It’s too bad because, unlike with COVID-19, the vaccines were already invented and developed, and same with the antiviral drug,” he said. “We just had too many lapses in our system.”

The lapses include an expiring batch of vaccine doses in reserve and a lack of alacrity in replacing them. So far, Illinois, New York and California have declared a state of emergency over the virus.

Hotez is optimistic the virus can still be contained since there are no asymptomatic cases.