Boston Mayor Thomas Menino declared a public health emergency on Wednesday in the face of a growing flu outbreak that so far has killed 18 people in Massachusetts and infected ten times as many Boston residents as last year's outbreak.

Across the United States, this year's flu outbreak has been more severe than usual. The U.S. Centers for Disease Control and Prevention reported that the proportion of people visiting their doctors for flu-like illnesses has doubled in the past four weeks.

Boston plans to make the flu vaccine available for free to residents of the city who have not yet received the inoculation.