Jan. 9, 2013 -- An early and nasty flu season has prompted a public health emergency in Boston, where health officials say 700 people have been diagnosed with the cold-weather virus. Four Bostononians -- all elderly -- have died from flu.

"This is the worst flu season we've seen since 2009, and people should take the threat of flu seriously," Boston Mayor Thomas Menino said in a statement.

This time last year the city had seen only 70 cases of influenza, The Associated Press reported. And with flu activity likely to extend into March or even April, the number will only grow.

Menino said the city is working with health care centers to offer free flu vaccines, and he urged anyone with flu-like symptoms to stay home from work or school.

"This is not only a health concern, but also an economic concern for families," he said in the statement. "I'm urging residents to get vaccinated if they haven't already."

Eighteen people have died from flu in Massachusetts, one of 41 states battling widespread influenza outbreaks. Emergency rooms across the country have been overwhelmed with flu patients, turning away some of them and others with non-life-threatening conditions for lack of space.

The proportion of people seeing their doctor for flu-like symptoms jumped to 5.6 percent from 2.8 percent in the past month, according to the CDC.

Northwestern Memorial Hospital in Chicago reported a 20 percent increase in flu patients every day. Northwestern Memorial was one of eight hospitals on bypass Monday and Tuesday, meaning it asked ambulances to take patients elsewhere if they could do so safely.

Dr. Besser's Tips to Protect Yourself From the Flu

Most of the hospitals have resumed normal operations, but could return to the bypass status if the influx of patients becomes too great.

"Northwestern Memorial Hospital is an extraordinarily busy hospital, and oftentimes during our busier months, in the summer, we will sometimes have to go on bypass," Northwestern Memorial's Dr. David Zich said. "We don't like it, the community doesn't like it, but sometimes it is necessary."

A tent outside Lehigh Valley Hospital in Salisbury Township, Pa., was set up to tend to the overflowing number of flu cases.

A hospital in Ohio is requiring patients with the flu to wear masks to protect those who are not infected.

State health officials in Indiana have reported seven deaths. Five of the deaths occurred in people older than 65 and two younger than 18. The state will release another report later today.

Doctors are especially concerned about the elderly and children, where the flu can be deadly.

"Our office in the last two weeks has exploded with children," Dr. Gayle Smith, a pediatrician in Richmond, Va., said

It is the earliest flu season in a decade and, ABC News Chief Medical Editor Dr. Besser says, it's not too late to protect yourself from the outbreak.

"You have to think about an anti-viral, especially if you're elderly, a young child, a pregnant woman," Besser said.

"They're the people that are going to die from this. Tens of thousands of people die in a bad flu season. We're not taking it serious enough."