If you haven’t had your flu shot, get it now.

That’s the message city officials were trying to impress upon the public as cases of illness have spiked this flu season. In Boston, 700 people have become sick with the flu since Oct. 1, which is 10 times more than the number of people who were sick during the entire previous flu season. Four of the 18 people who have died from flu-related causes across Massachusetts are from Boston, officials said.

Those numbers prompted Mayor Thomas Menino and city health officials to declare a public health emergency Wednesday.

“We have a terrible problem on our hands with lots of people sick with the flu,” said Dr. Barbara Ferrer, the executive director of the Boston Public Health Commission.

The strain of influenza that is predominately circulating in Boston is Type AH3 strain, which is a likely reason why this flu season is so severe, said Dr. Anita Barry, the director of infectious diseases at the BPHC.

“Typically the Type AH3 viruses are associated with more severe illness with influenza seasons that start earlier and that may go on a little bit longer and may involve more people,” said Barry.

That strain is covered by the vaccine, Barry said.

City health officials said there are thousands of people in Boston who have still not a flu vaccine this season.

Health officials said it is not too late to get a flu shot and urged people to take advantage of the free shots that will be offered this weekend in Boston in an effort to combat the spread.

“Being young and healthy does not protect you from getting the flu,” Ferrer said.

Menino said he is also concerned for the city’s lowest wage earners who may have little or no time off to care for themselves or their children.

“This is an economic issue besides a health issue,” he said.

Health officials said the best way for the flu to spread is by close respiratory contact, especially in crowded places. With that in mind, the MBTA is also taking steps to keep passengers as healthy as possible. Announcements about ways to avoid the flu are being made in stations and are being displayed on electronic signs.

Stay healthy

Tips to avoid getting sick or spreading germs:

*Wash your hands often with soap and water for 20 seconds, especially after coughing or sneezing. If water is not nearby, use an alcohol-based hand cleaner.

*Try not to touch your eyes, nose or mouth. Germs can spread this way.

*As much as possible, avoid close contact with people who are sick.

*If you have a fever or feel ill, stay home.

Free shots

The city is offering free flu shots to stop the spread. Below are some of the health centers that will offer the shots, but for more information visit www.bphc.org or call 617-534-5050.

*Dorchester House, 1353 Dorchester Ave., Saturday between 9 a.m. and noon.

*South Boston Community Health Center, 409 West Broadway, Saturday between 9 a.m. and 1 p.m.

*Fenway Health Center, 1340 Boylston St., Saturday between 9 a.m. and 1 p.m.