Update: influenza activity--United States, 2003-04 season.

This report summarizes influenza activity in the United States during September 29, 2003-March 27, 2004, and updates the previous summary. This report also summarizes human infections with avian influenza viruses related to poultry outbreaks in North America. Preliminary data collected through CDC influenza surveillance indicate that national influenza activity peaked during late November-December. The most frequently isolated viruses were influenza A (H3N2), and approximately 87% of these were similar to the drift variant A/Fujian/411/2002.