The two lists have some differences, noted Jean B. Patel, a C.D.C. specialist in drug-resistant bacteria who consulted with the W.H.O. For example, N. gonorrhoeae was a higher priority on the C.D.C. list because it is hard to treat, even though it rarely kills; the W.H.O. list focused more on fatal infections.

The C.D.C. does not consider H. influenzae — better known as Hib — to be as big a threat as the W.H.O. does, she said, because nearly all American babies get Hib shots, which prevent it. In poor countries, according to a 2009 study, more than 300,000 children a year die of Hib-related meningitis or pneumonia.

Some bacteria resistant to all known antibiotics have been found. They are rare and, thus far, usually strike patients whose immune systems are weak. But once they take hold, they are virtually unstoppable, and victims usually die.

It’s useful for the W.H.O. to set global research priorities because drug-resistant strains are not evenly spread around the world. Some strains are more common on some continents, although jet travel and medical tourism are making most spread worldwide.

Strains can even vary within hospitals.

For example, the antibiotics used in a transplant unit often differ from those used in neonatal intensive care, so different resistant bacteria may circulate, said Dr. William Schaffner, head of preventive medicine at Vanderbilt University Medical Center.

Infants in eastern Tennessee, he said, had more antibiotic-resistant ear infections than those in the state’s western half or in many other states. Doctors there are “exuberant prescribers,” he said, which drives antibiotic resistance.

Resistance to carbapenems, which had been reliable “last line of defense” drugs, has developed recently among pathogens, as a gene first found in India in 2008 — and named NDM, for New Delhi metallo-beta-lactamase — spread around the world.