A cold typically starts with a runny nose, or a scratchy throat, and then progresses to congestion, cough and feeling yucky. Young children may run a fever during the first day or two of a cold, and older children just "feel warm" but typically don't run much of a temperature. Despite feeling yucky you can keep going with a box of Kleenex at your side. Most children (and adults) are not bedridden with a cold.
Not so with flu, it hits you like a steam roller!! You suddenly feel sick and it all happens at once. Fever, chills, cough, congestion, sore throat, body aches all attack at the same time. From babies through teens, you can just look at them and know they are sick. The fever with the flu is usually higher and lasts longer than a cold. Flu even makes toddlers sit still and bed is usually the only place a teen wants to be. Adults just wish they could roll over and "fade away" for a few days (but parental duties continue to call, especially if the entire family gets the flu.)
A cold usually lasts anywhere from 7 - 14 days, but you feel the worst for the first few days and are then just bothered by the congestion and cough. (Read old blogs on colds). On the other hand, the flu usually lasts anywhere from 3 -7 days with fever, body aches, a terrible cough and congestion. The cough from the flu seems to go on for weeks. Children often run temperatures anywhere from 101 - 104 and are pretty miserable. Adults feel terrible with any real fever! In most cases the flu is entirely an upper respiratory virus. Occasionally a young child might vomit with the flu but that is not the typical presentation.
Although I know that flu comes every year, I cannot predict if it will last 3 days or 5 days or even 7 days until you actually begin to see cases. I can remember some years past when many children ran high fevers for 7 days with the flu during a particularly bad flu season. Best advice is to get the vaccine before we even start to see influenza! It is not an illness that you can "brush off", it sticks like glue and you know when you have it!