Everybody knows strep throat is as common as backpacks among school children, but one of the surprises in a new study of Pittsburgh students is that it isn't even more common. Elementary students "are sitting in a sea of strep," said Dr. Judith Martin, an infectious disease specialist at Children's Hospital who led a four-year study of the prevalence of group A Streptococcus bacteria in a Pittsburgh private school. The researchers report in this month's issue of the journal Pediatrics that six out of every 10 students tested positive for strep at least once during a school year. But just...