(CNN) – Flu continues to bring misery across the US, with all but four states showing high or very high levels of activity as a new virus strain called subclade K continues to spread.

By another measure – visits to the doctor for fever plus a cough or sore throat, which are common flu symptoms – the US is at its highest level of respiratory illness since at least the 1997-98 flu season, according to data published Monday by the US Centers for Disease Control and Prevention. (Read More)