Instances of a threatening superbug that scientists are calling the “phantom menace” are rising in the U.S., officials reported Thursday.

This particular strain of bacteria is a type of Carbapenem-resistant Enterobacteriaceae (CRE), which are dangerous because they typically are highly antibiotic-resistant and have steep mortality rates, the Centers for Disease Control and Prevention (CDC) said. According to the CDC, this CRE carries a plasmid with an enzyme that can break down certain antibiotics, which it can transfer to other bacteria in the body, the Washington Post explains.

But the “phantom menace” earned its name because it has gotten less attention than other CREs, since its antibiotic resistance is relatively lower than other strains, and because it can escape detection from scientists.

While there was only one case of the superbug in 2010, there have been 11 per year in 2013, 2014 and 2015. The CDC reported a total number of at least 43 patients in 19 states between June 2010 and August 2015.

“This is a tricky drug-resistant bacteria, and it isn’t easily found,” CDC Director Thomas Frieden told the Post. “What we’re seeing is an assault by the microbes on the last bastion of antibiotics.”

[The Washington Post]

This post originally appeared on Time.com.