Holiday travel and winter weather also contribute to the spread of the flu virus.
Flu season started early this year, and already doctors are seeing a spike in patients with the flu.
At the Midtown Medical Center, 35 patients tested positive for the virus in October and in the first two weeks of November there were almost 130 positive flu tests.
Knowing what to look for can help stop you from spreading the flu.
"You have high fevers, headaches, body aches, general malaise," says Dr. Jonathan Velasquez, E.R. physician at Midtown Medical Center. "And respiratory symptoms like, sore throat runny nose and cough."
Some great tips for staying healthy are washing your hands frequently, wiping surfaces daily, and covering your mouth when you cough.
The best way to do that is coughing into the inside of your elbow or a tissue -- not your hands.