The U.S. Centers for Disease Control and Prevention in 2013 released a list of drug-resistant bacterial and fungal infections, labeling each as “urgent,” “serious” or “concerning,” reflecting how dangerous they are, how prevalent and how difficult to treat. Most bacteria are classified as either Gram positive or Gram negative, depending on how they react to a laboratory staining technique. While the Gram-positive bugs methicillin-resistant Staphylococcus aureus (MRSA) and Clostridium difficile are the most well-known drug-resistant bacteria, many Gram-negative species are particularly hard to treat because they have an extra outer membrane that shields them from drugs.





Clostridium difficile

Prolonged use of antibiotics can allow this common intestinal inhabitant to explode into a lethal infection as the drugs kill off its beneficial rivals in the human gut. Spread via hospital surfaces and human contact, C. difficile most often affects the elderly. It causes severe diarrhea and can damage the colon, and it has become very difficult to treat.

CDC threat category: Urgent

CDC death estimate: 15,000

Statistical uncertainty of CDC estimate: 7,600 to 20,000

Number of states requiring reporting of disease: 17

Number of states requiring reporting of deaths: 3

Gram positive

Carbapenem-resistant Enterobacteriaceae (CRE)

Drug-resistant members of this family of Gram-negative bacteria are spread largely in healthcare settings. Strains of some species in the group, including Escherichia coli, Enterobacter aerogenes, Enterobacter cloacae and Klebsiella pneumoniae, are resistant to antibiotics called carbapenems, considered one of the last lines of defense against such infections. When carbapenems fail, doctors are often forced to turn to colistin, a decades-old drug that can have toxic side effects – and some CRE strains are already showing resistance to that drug, too.

CDC threat category: Urgent

CDC death estimate: 610

Statistical uncertainty of CDC estimate: 180 to 1,200

Number of states requiring reporting of disease: 22

Number of states requiring reporting of deaths: 6

Gram negative

Drug-resistant Neisseria gonorrhoeae

Some strains of this sexually transmitted infection, which can cause painful urination and inflammation in the pelvis, have developed resistance to the drugs commonly used to treat it.

CDC threat category: Urgent

CDC death estimate: Fewer than 5

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Multidrug-resistant Acinetobacter

Acinetobacter baumannii and other members of this genus are typically found in soil and water, and can survive on human skin and on medical equipment. They can cause pneumonia, urinary tract infections and serious blood or wound infections.

CDC threat category: Serious

CDC death estimate: 500

Statistical uncertainty of CDC estimate: 140 to 920

Number of states requiring reporting of disease: 6

Number of states requiring reporting of deaths: 2

Gram negative

Drug-resistant Campylobacter

This bug infects humans through contaminated milk, water or food – especially poultry – causing diarrhea, cramps and fever.

CDC threat category: Serious

CDC death estimate: 28

Statistical uncertainty of CDC estimate: 0 to 120

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Fluconazole-resistant Candida

Candida is a fungus, not a bacterium. The CDC included it on its list because strains of the fungus are increasingly showing resistance to the drugs commonly used to treat it. Candida is present in many people without doing harm, but it can cause serious infections in patients with weakened immune systems or if introduced into the bloodstream.



CDC threat category: Serious

CDC death estimate: 220

Statistical uncertainty of CDC estimate: 65 to 430

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Extended spectrum β-lactamase producing Enterobacteriaceae (ESBLs)

These bacteria, including strains of Escherichia coli and Klebsiella pneumoniae, produce an enzyme that destroys many antibiotics. They most often manifest as urinary tract infections, but can also cause serious bloodstream and lung infections. They are spread through improperly washed hands, surfaces and medical equipment. Some of the ESBL E. coli strains are also foodborne.



CDC threat category: Serious

CDC death estimate: 1,700

Statistical uncertainty of CDC estimate: 500 to 3,300

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Vancomycin-resistant Enterococcus (VRE)

Enterococci normally live in human intestines and the female genital tract without issue, but they can lead to serious infection when they spread through urinary or intravenous catheters, or enter the bloodstream. Some strains have developed resistance to vancomycin, one of the most powerful antibiotics available.



CDC threat category: Serious

CDC death estimate: 1,300

Statistical uncertainty of CDC estimate: 390 to 2,600

Number of states requiring reporting of disease: 10

Number of states requiring reporting of deaths: 1

Gram positive

Multidrug-resistant Pseudomonas aeruginosa

This pathogen thrives in moist environments and mostly affects hospital patients, especially those using mechanical ventilation or catheters or with surgical or burn wounds. The Gram-negative bacteria are exceptionally difficult to treat as they have developed resistance to multiple classes of drugs, in addition to their broad natural resistance.

CDC threat category: Serious

CDC death estimate: 440

Statistical uncertainty of CDC estimate: 130 to 850

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Drug-resistant non-typhoidal Salmonella

Non-typhoidal Salmonella is a common foodborne pathogen that causes more dangerous infection when it is resistant to common antibiotics. It causes severe, sometimes bloody diarrhea, cramps and fever.

CDC threat category: Serious

CDC death estimate: 40

Statistical uncertainty of CDC estimate: 0 to 120

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Drug-resistant Salmonella typhi

A member of the Enterobacteriaceae family, these bacteria spread through contaminated food or water or through person-to-person contact. Typhoid fever, rare in developed countries, can lead to serious health complications and death if untreated.

CDC threat category: Serious

CDC death estimate: Fewer than 5

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Drug-resistant Shigella

Shigella mostly affects young children and is spread through hand contact, food or water. In the U.S., a drug-resistant strain of the infection, which causes painful diarrhea, has been spread largely by travelers and spreads especially quickly in childcare settings and among homeless people and gay and bisexual men.

CDC threat category: Serious

CDC death estimate: Fewer than 5

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram negative

Methicillin-resistant Staphylococcus aureus (MRSA)

S. aureus is a once-routine infection that has become resistant to several powerful antibiotics. It most often occurs as a localized skin infection, but can become deadly if it enters the lungs or bloodstream, often through surgery or medical equipment. It is widely present in both healthcare settings and among the general population.

CDC threat category: Serious

CDC death estimate: 11,000

Statistical uncertainty of CDC estimate: 8,000 to 17,000

Number of states requiring reporting of disease: 27

Number of states requiring reporting of deaths: 4

Gram positive

Drug-resistant Streptococcus pneumoniae

S. pneumoniae can cause pneumonia; ear, sinus and bloodstream infections; and meningitis. Some strains are resistant to multiple drugs, which can be especially dangerous to young children, the elderly and HIV patients. It is spread person-to-person, often in childcare and healthcare facilities.

CDC threat category: Serious

CDC death estimate: 7,000

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram positive

Drug-resistant tuberculosis

Mycobacterium tuberculosis is spread through the air and usually infects the lungs, but can also infect organs such as the brain or kidneys. If caught early, the infection is largely treatable, but drug-resistant strains have emerged over the years. Some do not respond to many types of antibiotics and can be deadly in immunocompromised patients, such as those with HIV.

CDC threat category: Serious

The CDC conducts active tuberculosis surveillance in all 50 states and Washington, D.C. In 2011, 50 people died of drug-resistant tuberculosis.

Vancomycin-resistant Staphylococcus aureus (VRSA)

These S. aureus infections are resistant to vancomycin, one of the most powerful antibiotics available. They most often occur as skin infections. They can become deadly if they manifest as bloodstream or lung infections, especially in people with pneumonia or on ventilators.

CDC threat category: Concerning

CDC death estimate: Fewer than 5

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: 40

Number of states requiring reporting of deaths: 14

Gram positive

Erythromycin-resistant Group A Streptococcus

Group A Streptococcus is the most common cause of strep throat in children and adults, and erythromycin is one of the most commonly used antibiotics to treat it. The infection is usually mild, but can sometimes be life-threatening. It is spread through contact with infected mucus or through skin wounds or sores.

CDC threat category: Concerning

CDC death estimate: 160

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram positive

Clindamycin-resistant Group B Streptococcus

In newborns, Group B Streptococcus is a common cause of sepsis, a potentially fatal blood infection. Adults are susceptible, too. The widespread use of antibiotics to prevent the infection in newborns has caused some strains of bacteria to develop resistance to the drug clindamycin.

CDC threat category: Concerning

CDC death estimate: 440

Statistical uncertainty of CDC estimate: Not calculated

Number of states requiring reporting of disease: N/A

Number of states requiring reporting of deaths: N/A

Gram positive



