"Let's say you have the flu, you get better, you think everything is all clear," Gupta says. "Then a couple days later, the fever comes back. That means you may now have a bacterial infection."

Does this mean the flu vaccine isn't working?

The flu vaccine is only about 60 percent effective, according to the CDC. How well the vaccine works can vary based on your age and health, as well as how good the "match" of the vaccine is to this year's flu strain.

Each year, the vaccine is made from three strains of the influenza virus that researchers suspect will be the most active. So far, according to the CDC, this year's vaccine matches well with the most predominant type of flu spreading in the United States, but is less well matched to the No. 2 type of virus.

The vaccine usually works best in young, healthy people, according to Dr. Bill Schaffner, chairman of the preventative medicine department at Vanderbilt University. But even if you do get the flu, the vaccine can protect against harsher complications.

Learn more about the flu vaccine's effectiveness Will the flu vaccine make me sick?

The virus used in the flu shot is a dead virus, Gupta says. "You cannot get the flu from a flu shot." But your body's immune system will react, and that can make you feel pain in your arm or feel less than 100 percent for a few days.

Is there a flu vaccine shortage?

While CNN has heard reports of vaccine shortages in some areas, manufacturers say there are vaccines available for anyone who wants to get a flu shot.

There was a temporary delay in new shipments of the antiviral medicine Tamiflu due to an increased demand, according to a spokeswoman. Tamiflu can help reduce the severity and length of flu symptoms if taken early.

I'm sold! Is it too late to get the vaccine?

In one word: no. Keep in mind that it takes about two weeks for antibodies to develop in your body and provide protection against the virus, the CDC says.

How can I avoid getting the flu in the first place?

It's disturbing to think about, but flu germs can spread up to 6 feet through coughs and sneezes. They can also live on surfaces for up to eight hours.

Dr. Jennifer Shu, a pediatrician and mother of two, recommends practicing good "social distancing." This means canceling parties or dates if someone is sick and avoiding large crowds (like at the movies or going out to dinner) if your community has been hit hard by the virus.

It's also a good idea to practice proper hand-washing -- for at least 20 seconds several times a day. And wiping handles, counters and remotes daily with hot soapy water or disinfectant will kill any germs that linger.