Flu is the common term for seasonal influenza, an infectious disease that occurs when certain species of viruses gain a foothold in the body’s respiratory system. Some forms of seasonal influenza produce only minor threats to human health, while others can produce severe threats or even kill affected individuals. According to the results of a new study published in May 2013 in JAMA Psychiatry, children exposed to influenza while in the womb apparently have significantly increased chances of developing bipolar disorder at some point during their adult lives. This finding echoes a previous study which indicates that prenatal exposure to influenza also significantly increases lifetime risks for developing schizophrenia. Flu Basics Flu viruses don’t stay the same from year to year. Instead, they can vary in ways large or small, and viruses that appear one year may or may not pose the same level of threat as viruses that appeared in the past. It is this changeable quality that leads doctors and researchers to refer generally to flu-related illnesses as seasonal influenza. The changeable nature of influenza viruses also accounts for the use of seasonal vaccines to combat the spread of flu outbreaks. These vaccines—which contain either dead copies of the seasonal virus or weakened forms of live virus—provide protection by stimulating the immune system and priming that system to fight off any live, full-strength virus copies that appear within the nose, throat, or lungs. A number of factors contribute to the degree of risk associated with any seasonal influenza outbreak, the Centers for Disease Control and Prevention explain. These factors include the specific strains of virus circulating in the general population during any given period of time, the severity of a flu virus’s effects on the human body, the amount of flu vaccine released to the public, the effectiveness of that vaccine in combating seasonal influenza, and the number of people who get vaccinated during flu season. Bipolar Disorder-Related Risks In the study published in JAMA Psychiatry, a multi-institution research team examined the lifelong medical histories of all of the children born within a single county in northern California in a seven-year period that started in 1959, 722 of these children were exposed to seasonal influenza while still in their mothers’ wombs. Of these exposed children, 92 developed some form of bipolar disorder during adulthood. The researchers compared this rate of bipolar disorder development to the rate found in a second set of 722 children from the same group whose mothers did not catch the flu during pregnancy. After making their comparisons, the authors of the study concluded that fetal exposure to seasonal influenza is linked to an almost 300 percent increase in a person’s lifetime risks for bipolar disorder. They also concluded that fetal influenza exposure is linked to an almost 500 percent increase in a person’s lifetime risks for an especially severe subtype of bipolar disorder that combines the classic symptoms of mania and depression with psychotic symptoms normally associated with the presence of schizophrenia, or any one of a group of schizophrenia-related disorders. While influenza exposure increases bipolar-related risks at all stages of pregnancy, mildly greater risks appear when the flu strikes during a pregnancy’s final six months. Schizophrenia-Related Risks In an earlier study that included many of the same participants as the new study published in JAMA Psychiatry, the same team of researchers examined the connection between fetal exposure to seasonal influenza and the chances of developing schizophrenia during adulthood. They concluded that children born to women who catch the flu during the first four-plus months of pregnancy have 200 percent greater lifetime schizophrenia risks than children not exposed to the flu while in the womb. The authors of both studies believe that the increased risks for both bipolar disorder and schizophrenia reflect the common genetic origins of these mental disorders, as well as their highly similar patterns of development over time. Considerations The authors of the study published in JAMA Psychiatry believe that pregnant women should protect themselves from seasonal influenza by getting vaccinated and staying away from people infected with a current influenza strain. They also believe that women seeking to get pregnant should take the same precautions. Apart from any considerations regarding bipolar disorder risks or schizophrenia risks, pregnant women have increased chances of developing flu-related complications, the CDC notes. Currently, only a minority of pregnant women in the U.S. receives seasonal flu vaccinations.