Of the 18 bacteria described in a Centers for Disease Control and Prevention report on antibiotic resistance, three are classified at the highest threat level: urgent. The report defines those antibiotic threats as “immediate public health threats that require urgent and aggressive action.” Here’s the rundown:

• Clostridium difficile: This bacteria triggers life-threatening diarrhea, the report says, and typically infects individuals who had both recent medical care and antibiotics. Antibiotic resistance is not currently a barrier to treating C. difficile infections, but the bacteria spreads quickly because of its natural resistance to many treatments for other infections. The CDC says C. difficile causes 250,000 infections each year that require hospitalization or affect patients already hospitalized and results in 14,000 deaths annually. It’s also linked to at least $1 billion per year in excess medical costs.

• Carbapenem-resistant Enterobacteriaceae: Infections from CRE bacteria that are untreatable or difficult to treat are increasing among patients in medical facilities, according to the report. CRE have grown resistant to all or almost all available antibiotics and almost 50 percent of hospital patients who get bloodstream infections from the bacteria are killed by them. Luckily, those infections are a minority.

The CDC estimates that CRE causes more than 9,000 drug-resistant infections annually and about 600 deaths. “It is truly a nightmare bacteria,” CDC Director Thomas Frieden said.

• Neisseria gonorrhoeae: This bacteria produces gonorrhea, a sexually transmitted disease that spreads easily, the report says. It has a disproportionate effect on sexual, racial and ethnic minorities and results in severe reproductive complications.

The CDC estimates that there are 820,000 cases of gonorrhea each year and 246,000 cases of drug-resistant infections.