Update: influenza activity--United States and worldwide, May-October 2004.

May-October 2004, influenza A (H3N2) viruses circulated worldwide and were associated with mild-to-moderate levels of disease activity. Influenza A (H1N1) and B viruses were reported less frequently. In North America, isolates of influenza A (H3N2), A (H1N1), and B were identified sporadically. This report summarizes influenza activity in the United States and worldwide during May-October 2004. Influenza activity in North America typically peaks during December-March.