The flu is a respiratory illness caused by viruses that spread from person–to–person. The period between becoming infected with the virus and becoming ill is usually 1 to 4 days.
Flu viruses spread by coughing and sneezing respiratory droplets into the air. Flu usually spreads from close person–to–person contact, though touching something with flu virus on it and then touching the mouth, eyes, or nose sometimes infects people. The virus can live for as long as two hours on surfaces like doorknobs, phones, and furniture.
Because symptoms do not develop as soon as one is infected, a person can pass on the flu to someone else before feeling ill or showing any symptoms.