The alarming surge in cases of severe tick-borne diseases underscores how quickly the risks of infection are changing. Several species of ticks — the lone star, the Gulf Coast tick and the infamous blacklegged tick (also known as the “deer tick”), which spreads Lyme disease — were once regarded as mere nuisances confined to relatively small regions of the United States. But warming temperatures and changes to landscapes have combined to help them expand their range and bring a host of pathogens with them. Some are manageable; others cause illnesses that are severe and untreatable.