Update, influenza activity--United States and Canada.
D000293:Adolescent; D000328:Adult; D002170:Canada; D002648:Child; D004196:Disease Outbreaks / Q000453:epidemiology*; D006801:Humans; D009980:Influenza A virus / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology*; D008875:Middle Aged; D014481:United States