Serious and fatal bouts of malaria in the United States are a greater problem than has been previously reported, according to a new study. Most appear to be in immigrants who have made summer or Christmas visits to their home countries without taking precautions against infection.

The typical victim appears to be a man ranging in age from 20 to 50 who is from Africa or the Caribbean, said the lead author, Diana Khuu, an epidemiologist at the Fielding School of Public Health at the University of California, Los Angeles.

But among the hospitalized women, an unusually high number — 14 percent — were pregnant. Because pregnancy lowers immune defenses, malaria can be lethal to both mother and fetus.

Although the study was based on hospital data rather than interviews with patients, the authors suspect that many of the victims grew up in malarial areas, developed immunity in childhood from repeated infections, and then did not realize that their childhood immunity had disappeared after years in the United States.