Elva Etienne/Getty ImagesWalking pneumonia infections rose nationwide as kids returned to school this fall.The infection can be tricky to diagnose, and testing is expensive.Doctors may choose different antibiotics to try first, working on a case-by-case basis.A common infection known as "walking pneumonia" has been on an uptick across the US in recent months, especially among kids and teens.The respiratory illness caused by mycoplasma pneumoniae bacteria is often called walking pneumonia because it tends to be milder than other lung infections.