Tennessee and Georgia are two of 36 states where the Centers for Disease Control and Prevention says the flu is widespread.

Federal health officials warned earlier this month that much of the influenza virus circulating in the United States this year has mutated, and that this year's vaccine does not provide much protection.

Vaccines are cooked up months in advance, and flu viruses often mutate. Flu experts say that a flu shot is still the best way to protect yourself — there are many different flu strains covered.

"Influenza vaccination is still the best way to prevent the flu — the vaccine is safe and flu kills," Dr. Carol J. Baker, executive director for the Center for Awareness and Research at Texas Children's Hospital, told NBC News.

READ MORE | Channel 3 Flu Tracker

Health officials recommend washing your hands often, covering your mouth, staying at home if you're sick and getting the flu vaccine. Even though the vaccine is not 100 percent effective, doctors said some protection is better than none at all.

Most experts think that flu viruses are spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth or nose.

The CDC's website also says that most healthy adults may be able to infect other people beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick.

Children may pass the virus for longer than 7 days. Symptoms start 1 to 4 days after the virus enters the body. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Some people can be infected with the flu virus but have no symptoms. During this time, those persons may still spread the virus to others.