This is a remarkably persistent belief, but there's really no truth to it. The CDC says that the timing of flu outbreaks is unpredictable from year to year, but data show that between the 1982-83 and 2011-12 seasons, the peak month most often has been February, which topped the list in 14 of the years studied, or 47 percent of the time. And the next two months when flu infections most often peak are Jan. and March. December comes in after that, with that month leading the year in infections in only four years, or about 13 percent of the time [source: CDC].
The mind-boggling precision of that bit of data may lead you to wonder: How do public health officials even know that sort of stuff? Government health agencies in both the U.S. and other countries have set up a global system for tracking infectious diseases, including the flu. In the U.S., that work is carried out by the National Respiratory and Enteric Virus Surveillance System, a network of 60 laboratories scattered around the country that test nasal and throat cultures from patients who show up at hospitals and doctors' offices complaining of symptoms. That network works in coordination with an even larger warning system, the U.S. Outpatient Influenza-like Illness Surveillance Network, which includes more than 3,000 healthcare providers in all 50 states. They track all cases of patients with fevers of more than 100 degrees Fahrenheit (38 degrees Celsius) and coughs and/or sore throats [source: CDC].