Cold

Lasts for three to fourteen days. Most often in the winter but can possibly occur at any time. Symptoms take a few days to appear after infection with the virus. Cold is often accompanied by cough, sore throat, and Nasal secretions that are usually in the form of yellow mucus. Sometimes there is body aches and fatigue or mild fever.

Flu

Flu symptoms are usually more severe than cold symptoms and come on quickly. Common symptoms of flu include sore throat, fever, headache, muscle aches and soreness, congestion, and cough. Occasionally, vomiting and diarrhea may occur as in the case of swine flu. Most flu symptoms gradually improve over two to five days, but it's not uncommon to feel run down for a week or more.

Allergy

Can last for Days to months -- as long as you are exposed to the allergen. It happens any time of the year -- although the appearance of some allergens is seasonal. It is often accompanied by itchy watery eyes and clear secretions from the nose. Sometimes it is accompanied by cough, fatigue or sore throat. Body aches and fever are not considered as allergy symptoms.

Although there are some differences, cold, flu and allergy symptoms overlap quite a bit. The most important difference is that colds usually don't last longer than 14 days. If you still have symptoms after two weeks, see your doctor. These may be allergy symptoms or a sign of another problem like a sinus infection for instance.