They have different causes. You get a cold when a tiny living thing called a virus gets into your body. There are hundreds of different types that can get you sick.
Once a cold virus gets inside you, your immune system, the body's defense against germs, launches a counter-attack. It's this response that brings on the classic symptoms like a cough or stuffed up nose.
The viruses that cause colds are contagious. You can pick them up when someone who's infected sneezes, coughs, or shakes hands with you. After a couple of weeks, at the most, your immune system fights off the illness and you should stop having symptoms.
It's a different story with allergies. They're caused by an overactive immune system. For some reason, your body mistakes harmless things, such as dust or pollen, for germs and attacks them.
When that happens, your body releases chemicals such as histamine, just as it does when fighting a cold. This can cause a swelling in the passageways of your nose, and you'll start sneezing and coughing.
Unlike colds, allergies aren't contagious, though some people may inherit a tendency to get them.
Differences Between Colds and Allergies
Take stock of your symptoms and how long they last to help you decide what's causing your trouble.
How Long It Lasts
Days to months -- as long as you're in contact with the allergy trigger
When It Happens
Most often in the winter, but possible at any time
Any time of the year -- although the appearance of some allergy triggers are seasonal
When It Starts
Symptoms take a few days to appear after infection with the virus
The most important difference is that colds usually don't last longer than 14 days. So see your doctor if you still have symptoms after 2 weeks. These may be allergy symptoms or signs of another problem.
U.S. Department of Health and Human Services: "Is it a cold or an allergy?"
Asthma and Allergy Foundation of America: "Flu/Cold or Allergies?"
American Academy of Asthma, Allergy, and Immunology: "Tips to Remember: What are Allergy Shots?"
News release, FDA.