Colorado has experienced a surge in walking pneumonia this fall, most notably among very young children who aren’t usually as susceptible to the illness, according to health officials. Bacterial infections that cause the mild lung infection reemerged nationwide this year for the first time since the start of the pandemic, peaking in August after increasing sharply since the spring, according to the Centers for Disease Control and Prevention. In Colorado, walking pneumonia cases peaked a little later, in September, but the illness is expected to continue spreading at a high rate into the new year, said Dr.