CHICAGO — Three people returned to the U.S. from India earlier this year infected with newly described “”superbugs”” that are highly resistant to antibiotics, according to the U.S. Centers for Disease Control and Prevention.

All three confirmed U.S. cases — one in California, one in Illinois and one in Massachusetts — involved people who had received medical care in India.

There, several kinds of bacteria are spreading that carry a gene called NDM-1, which makes a carbapenem-destroying enzyme. Carbapenems are key antibiotic weapons against these “”Gram-negative”” bacteria, which are already challenging to treat.

Infectious disease experts have described the situation as a potential nightmare scenario in which other bacteria acquire the gene, rendering an entire class of antibiotics useless against them.

The Illinois patient recovered, and there is no evidence the infection was transmitted to other people, said Illinois Department of Public Health spokeswoman Melaney Arnold, who released no other details about the case.

The three U.S. cases involved three different bacteria that remain susceptible to the antibiotics colistin, polymixin and tigecycline, said Karen Bush, an Indiana University professor and widely known expert on resistance in bacteria.

But “”one of the problems is that use of these antibiotics can also eventually result in resistance,”” said Bush, who spoke about resistant bacteria Monday at the Interscience Conference on Antimicrobial Agents and Chemotherapy in Boston.

There are no drugs in Phase 3 clinical trials that work against the NDM-1 mechanism, said Bush. But, she said, there are some promising compounds in earlier stages of development.

Medical epidemiologist Dr. Alex Kallen of the CDC warned against panic. Although the NDM-1 mechanism is new, he said, other bacteria already found in the state also can evade carbapenems.

“”That is not to downplay this,”” he said. “”It is important, but this particular mechanism is just one of many that can cause this kind of pattern.””