SPOKANE, Wash. -- If you are sick, it may be difficult to determine if you have the flu, common cold or stomach virus.

The flu and cold are both respiratory illnesses but they are caused by different viruses.

The flu affects the lungs and sinuses. Flu symptoms come on suddenly and can include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches and exhaustion or fatigue.

A cold, comes on gradually. Cold symptoms are usually milder than the symptoms of flu. People with colds are more likely to have a runny or stuffy nose.

Gastroenteritis, is known as a stomach virus and is a completely different virus from the seasonal flu. It is most often caused by norovirus, known as the cruise ship illness, rotavirus or food-borne bacteria. Its symptoms include, vomiting, diarrhea, stomach pain and cramping, and occasional fever.

Adults rarely experience vomiting and diarrhea with the flu. Vomiting and diarrhea are more common in children with the flu. A stomach virus can be miserable but is typically not as serious as the flu.

The flu symptoms may last anywhere from two to 10 days. If you have the flu, start feeling better for a few days and then develop worse symptoms with a higher fever, call your doctor. You may need a different treatment depending on your diagnosis.