The flu is a highly contagious virus that attacks the upper respiratory system. According to Kid'sHealth.org, the illness generally has a quick onset and you may have few symptoms prior to getting sick. The preliminary symptoms you do have may be mild or easily confused for something else. In order to determine if you are coming down with the flu, pay attention to the subtle signs.

Pay attention to your feelings. You may experience a general feeling of malaise accompanied by fatigue or even a mildly depressed mood.

Check for swollen glands. When you're coming down the flu, the lymph nodes in your neck and under your arms may swell. The swelling may not be visible but you should be able to feel it by touching your neck just below the jaw, and under your arms. You may also feel a fullness in your neck and have difficulty swallowing.

Look for aches and pains. Your muscles may feel stiff and sore, as if you had just worked out. You may also feel mild pain in your neck and under your arms, in the location of your lymph nodes.

Pay attention to your sinuses. Congestion is one of the symptoms of the flu 3. Your sinuses may swell before you become sick, and you may feel pressure behind your eyes. You may also experience post-nasal drip and a scratchy throat.