The number of people sick with the flu is continuing to climb, and transmission is now the most intense it has been since the 2009 pandemic, Centers for Disease Control and Prevention officials said Monday.

The federal agency continued to track and respond to the epidemic despite the government shutdown. Its flu laboratories were working Monday, and “we are continuing to look at data we have received from states so that public-health officials can know about their community and influenza,” CDC Director Brenda Fitzgerald said...