As Americans head into the holiday season, a rapidly intensifying flu season is straining hospitals already overburdened with patients sick from other respiratory infections.

More than half the states have high or very high levels of flu, unusually high for this early in the season, the government reported Friday. Those 27 states are mostly in the South and Southwest but include a growing number in the Northeast, Midwest and West.

This is happening when children's hospitals already are dealing with a surge of illnesses from RSV, or respiratory syncytial virus, a common cause of coldlike sympto...