(WZZM)- The number of cases of the flu continues to rise with 18 states on the verge of epidemic levels according to the CDC.

All but a few states and the District of Columbia have widespread flu activity. That includes Michigan.

Hospitalizations are up and in many cases those are people suffering from complications from the flu and there are groups more at risk for those.

Dr. David Dobbie, an Infectious Disease Specialist from Spectrum Health is an expert on the flu.

"Anyone with chronic underlying medical conditions, obesity, diabetes, if they're pregnant, very young or very old, those are the people at highest risk of complications from influenza," said Dr. Dobbie.

Speaking of the very young,school startsMonday for thousands of West Michigan students.

With that comes a higher risk of the spread of the flu virus.

"Because of the cough and because children are very efficient at transmitting the virus and there's going to be a grouping of a lot of other susceptible children, that can really create an explosive situation in terms of how this virus can be transmitted,"said Dobbie.

There are a couple of other important things to remember about the flu and the vaccine.

It takes about two weeks for the vaccine to take effect in your body, but the incubation period for the flu is only a day or two.

So while it might seem like getting a shot now won't help you. it is still the best defence against the flu.

"In the most global sense, having the whole community focus on getting an immunization, which isn't going to help on Monday, but it can help if this outbreak of influenza continues over the next weeks and months as it may well do."said Dobbie.

Finally, local health departments, doctors offices and health clinics have an ample supply of the flu vaccine.

If you go to your doctor, your insurance should pay for the shot. If you go to a pharmacy, you may have to pay for it.

Pharmacists say the best advice is to call ahead to make sure they have shots available.