Both colds and the flu are contagious and are caused by viruses. However, the viruses that cause colds (for example, rhinoviruses) are not the same as those that cause the flu (influenza viruses). Although the typical incubation period for influenza is about one to four days, some adults can be contagious from about one day before onset of symptoms for up to two weeks
Other people who develop complications, such as pneumonia, may extend the contagious period for a week or two. For colds, most individuals become contagious about a day before cold symptoms develop and remain contagious for about five to seven days. Some children may pass the flu viruses for longer than seven days (occasionally for two weeks).
Colds are considered upper respiratory infections. The flu may also cause lower respiratory infections
For both the cold in the flu, early symptoms may be similar. Symptoms and signs include a cough, runny nose, and feeling tired. If you know you have had contact with someone with a cold or the flu in the past few days, you should suspect you may have become infected
However, flu symptoms generally are more intense than cold symptoms. People with flu can develop fever, body aches, chills, and headaches, and some develop nausea and vomiting. Cold symptoms are much milder and usually do not require medical care. However, if you suspect you have the flu, you should seek medical care. The flu often can be diagnosed with rapid tests available to most physicians.