Three people, who returned to the US from India earlier this year, have been infected with the "superbug" that are highly resistant to antibiotics, the US Centres for Disease Control and Prevention has said. All three confirmed US cases - in Massachusetts, California and Illinois - involved people who had received medical care in India. A person infected with the 'superbug' was treated earlier this year at Massachusetts General Hospital and isolated, a move that helped prevent the germ from spreading. The patient had recently travelled from India. The Illinois patient too recovered, and there is no evidence the infection was transmitted to other people. The US Centres for Disease Control and Prevention said the Massachusetts patient survived, as did the only other two US patients with infections. All three patients developed urinary tract infections that carried a genetic feature that made their cases harder to treat. The superbug, also known as NDM-1 — short for New Delhi metallo-beta-lactamase — allows bacteria to escape some of the strongest antibiotics available. "It leaves treating physicians with few treatment options," the Boston Globe quoted Alex Kallen, a CDC medical officer, as saying. All three of the US patients had been in India, and two underwent medical procedures in hospitals while they were there, Kallen said. The patient treated in Boston was an Indian citizen with cancer who had undergone surgery and chemotherapy in that country before coming to Massachusetts, Kallen added. Cases of NDM-1 infections have been reported in Asia, Europe and Canada. Experts have said the threat posed by the germs in the US is most acute in hospitals. "They don't cause infection in people walking down the street," said Dr Alfred DeMaria, top disease tracker for the Massachusetts Department of Public Health. "If somebody is in an intensive care unit on a ventilator with a tube in their trachea, they are at risk for these organisms. If someone has had extensive abdominal surgery with lots of open wounds, they are at risk." Only two antibiotics possess a measure of effectiveness against bacteria riddled with NDM-1, doctors said: an old drug called colistin, and tigecycline.