Biologists have long suspected that mutant genes pass the potential for schizophrenia from generation to generation but that they have to be turned on by something in the environment. Head injuries, maternal malnutrition, and rubella during gestation have all been found to increase a person’s risk of contracting the disease. But the leading candidate for an environmental trigger was only recently discovered: a dormant retrovirus incorporated into the human genome millions of years ago. Virologist Robert Yolken of Johns Hopkins School of Medicine found the retrovirus after examining [in] the spinal fluid of 35 schizophrenics. In 29 percent of those who had recently developed symptoms and in 7 percent of chronic cases, the retrovirus was active and generating RNA, the template for proteins. Yolken found no such signs in people free of schizophrenia.