Federal health officials on Wednesday linked the H1N1 flu epidemic to a sharp rise in the number of severe bacterial infections.
Anne Schuchat, a physician at the Centers for Disease Control and Prevention, said the national trend was "worrisome" but not unexpected.
"In previous pandemics, there has been an increase in pneumococcal infections in younger people," she said.
The illnesses are caused by Streptococcus pneumoniae, a microbe often carried in the nose and throat. While often benign, the bacterium can cause bloodstream infections, fatal pneumonia and meningitis.