After two relatively mild flu seasons, this year has quickly turned nasty, prompting Mayor Thomas M. Menino of Boston to declare a public health emergency Wednesday. Clinic waiting rooms are overflowing, and hospitals are scrambling to find beds for patients made miserable by high fevers, breathing problems, and dehydration.

The surge in cases comes despite a government recommendation three years ago that vaccination be expanded to everyone except infants under 6 months of age. Infectious disease specialists blame the particular virus strain that is circulating, which tends to cause worse illness, combined with the large percentage of people who have yet to be vaccinated.