Influenza (flu) is a contagious respiratory illness caused by influenza viruses which infect the nose, throat, and lungs. It can cause mild to severe illness and it affects all age groups, although children younger than five and adults over 65 and those with weakened immune system are most vulnerable.Signs and Symptoms .As you are probably wondering if there is any difference between flu and common cold, YES, there is a difference. The difference mainly lies with the onset, severityand duration of the flu symptoms over cold symptoms. Those of flu can emanate more quickly within hours and are severe while common cold symptoms are mild and can develop more gradually usually within a matter of days. Flu symptoms can last for 3-5 days while cold symptoms may take up to 10 days.