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Doctors still don't know what exactly causes schizophrenia, a devastating mental illness characterized by extremely disordered thinking. They're pretty sure that some kind of genetic predisposition is at work. But they also suspect that environmental triggers — particularly at critical moments during the brain's development before birth — play a role. That's why the results of a study published last week in the Archives of General Psychiatry are so intriguing. For the first time, researchers have direct evidence that exposure to influenza in utero is tied to a greater likelihood that an individual will someday develop schizophrenia.

That doesn't mean that the link between influenza and schizophrenia is airtight. "It's really important to duplicate these results," says Dr. Alan Brown, a psychiatrist at the New York State Psychiatric Institute in New York City, who led the study. But the new findings fit a pattern that has been emerging since researchers noticed a spike in cases of schizophrenia among people born in Denmark in 1957 during a major flu epidemic.

Brown and his colleagues looked at blood-serum samples taken from HMO patients who were pregnant between 1959 and 1966. Then they zeroed in on those women whose children later developed schizophrenia. The researchers discovered that the presence of influenza antibodies — a sure sign of infection — during the first half of pregnancy correlated with a threefold greater risk of schizophrenia. There was no correlation with influenza during the second half of pregnancy.

Why would exposure to influenza during pregnancy increase the risk of schizophrenia? No one knows. Perhaps the infection somehow damages the developing brain. Or the reason may have something to do with how influenza affects the mother's lungs, decreasing the amount of oxygen that can get to the fetus. But even if the link is real, it would account for just 14% of schizophrenia cases.

Developing influenza during pregnancy carries its own more immediate risks and often requires hospitalization. That is why the Centers for Disease Control and Prevention recommends that all women who are or plan to be pregnant during flu season get vaccinated starting in the fall. To avoid infection, it also helps to wash your hands regularly and steer clear of anyone who's sneezing.

• Christine Gorman