PARKER, Colo. — The Marker family opened their door on a recent evening to a woman dressed in purple, with a military attitude to cleanliness. Linda Holmes, who has worked as a technician with LiceDoctors for five years, came straight from her day job at a hospital after she got the call from a dispatcher that the Marker family needed her ASAP. According to those in the world of professional nitpicking, Pediculus humanus capitis, the much-scorned head louse, has returned. “It’s definitely back,” said Kelli Boswell, owner of Lice & Easy, a boutique where people in the Denver area can get delous...