For U.S. school districts, few issues matter more than student attendance. Each year more than 7 million students are chronically absent, at high cost. Student achievement, graduation rates, employment prospects, indeed success in life — all are linked to attendance. In most states, so is school funding. Among the many causes of absenteeism — poor grades, bullying, unstable housing — one in particular is often overlooked: illness. When students miss school because of a stomach bug, strep throat, or asthma, schools may reason, How much can we really do?