The World Health Organization is issuing a warning about a group of deadly bacteria: Recently, the WHO released its first-ever listof "priority pathogens," a list of antibiotic-resistant bacteria that the organization says pose the greatest threat to human health.

The list is divided into three categories: critical-, high- and medium-priority. Three pathogens made it into the critical-priority group. These bacteria are resistant to multiple antibiotics and pose a high risk to people in hospitals and nursing homes, the WHO says.

Multidrug-resistant bacteria, sometimes called "superbugs," are a critical priority because infections with these germs can be deadly, according to the WHO. For example, people who get infections from a type of multidrug-resistant bacterium called methicillin-resistant Staphylococcus aureus (MRSA) have a 64 percent greater chance of dying than people who contract the same infection in its nonresistant form, according to the WHO. [6 Superbugs to Watch Out For]

All of the top three pathogens on the list are resistant to a group of antibiotics called carbapenems. These antibiotics are sometimes referred to as "last resort" medications, because if they don't work, very few options are left.

"It is important the WHO take this on, because with travel and now widespread communication, an antibiotic-resistant organism … is going to get around the world pretty quickly," said Dr. Kenrad Nelson, a professor of infectious-disease epidemiology at the Johns Hopkins Bloomberg School of Public Health. Nelson was not involved in compiling the WHO's list.

Overall, the WHO's list is good, Nelson told Live Science. He noted, however, that he would have included the pathogen Clostridium difficile on the list. C. diff can occur in patients who receive antibiotics and is difficult to treat and get rid of completely, he said.

Here are the top three germs the WHO is worried about:

Carbapenem-resistant Acinetobacter baumannii

This bacterium can cause pneumonia, serious blood infections and other conditions, according to the Centers for Disease Control and Prevention.

A. baumanniioccurs primarily in hospitalized patients. It spreads through either person-to-person contact, or contact with a contaminated surface, the CDC says. Although the pathogen doesn't pose a big threat to healthy people, it's very dangerous for patients with compromised immune systems or chronic diseases, the CDC says.

Outbreaks of A. baumannii typically take place in hospital settings such as intensive care units (ICUs) or long-term health care facilities with sick patients, such as nursing homes, according to the CDC. [27 Devastating Infectious Diseases]

It's unclear how common this pathogen is in many countries around the world; however, A. baumannii is estimated to cause between 2 and 10 percent of multidrug-resistant bacterial infections in ICUs in Europe and the U.S., according to the WHO.

Carbapenem-resistant Pseudomonas aeruginosa

P. aeruginosa infections most often occur in the hospital. For patients with P. aeruginosa infections,pneumonia or infections following surgery can become extremely dangerous, and even life-threatening. But these bacteria can also live in hot tubs and swimming pools, and have been linked to serious ear infections and skin rashes, according to the CDC.

P. aeruginosainfections occur most often in hospitals; patients can become infected with the bacteria from contact with a breathing machine or a catheter, or through a surgical wound, according to the CDC.

The infection is most dangerous to those with weakened immune systems.

The CDC estimates that about 51,000 P. aeruginosa infections occur in health care settings in the U.S. each year; of these infections, more than 6,000 are from multidrug-resistant forms of the bacteria. About 400 deaths in the U.S. per year are linked to this infection, the CDC says.

Carbapenem-resistant Enterobacteriaceae

Infections with carbapenem-resistant Enterobacteriaceae (CRE)most often occur in hospitals or long-term health care settings, the CDC says. Similar to A. baumannii, CRE usually does not pose a risk to healthy people; rather, it is most dangerous to people with compromised immune systems, according to the CDC.

CRE can spread through person-to-person contact or through medical devices such as ventilators, the CDC says.

In a 2015 study published in the journal JAMA, researchers found that CRE affected approximately 3 in 100,000 people in the U.S. Of the 599 cases studied, 51 patients died.

Other concerning germs

In the other two categories on the priority-pathogens list, the WHO included germs that are resistant to certain antibiotics and those that cause diseases including gonorrhea and Salmonella food poisoning.

Six pathogens were included in the high-priority category, and three pathogens were listed in the medium-priority category. The six high-priority pathogens are: Enterococcus faecium, vancomycin-resistant; Staphylococcus aureus, methicillin-resistant, vancomycin-intermediate and resistant; Helicobacter pylori, clarithromycin-resistant; Campylobacter spp., fluoroquinolone-resistant; Salmonellae, fluoroquinolone-resistant; and Neisseria gonorrhoeae, cephalosporin-resistant, fluoroquinolone-resistant. The three medium-priority pathogens are: Streptococcus pneumoniae, penicillin-non-susceptible; Haemophilus influenzae, ampicillin-resistant; and Shigella spp., fluoroquinolone-resistant.

The WHO list was developed in collaboration with the Division of Infectious Diseases at the University of Tübingen in Germany. To determine which bacteria to include, researchers looked at a few factors, including how deadly the infections caused by the bacteria are, how resistant the bacteria are to existing antibiotics, how easily the bacteria spread, the number of treatment options available, and how preventable infections caused by the bacteria are, according to the WHO.

One of the main goals of the list is to drive more research into the development of new antibiotics and inspire governments to invest in this research and development, WHO officials said.

In addition, better prevention and the appropriate use of existing antibiotics are required in order to adequately address this threat, they added.

Indeed, "one issue is that one of the things that promotes antibiotic resistance is use of an antibiotic," Nelson said. "In general, antibiotics tend to be overused, and that's one of the things that leads to resistance."

Originally published on Live Science.