Flu shots protect against the flu, but not a cold. Different viruses cause the diseases. Pneumonia can be a complication of the flu, and commonly can be caused by either viruses or bacteria.

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The distinctions can be confusing, so here's a look at the symptoms for a cold, the flu and pneumonia and how they vary:

• FEVER

Cold: Rare in adults and older children; can be as high as 102 degrees Fahrenheit in infants and small children.

Flu: Usually 102 degrees Fahrenheit but can go up to 104 degrees Fahrenheit; lasts three to four days.

Pneumonia: May or may not be high; can be as high as 105 degrees Fahrenheit.

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• HEADACHE

Cold: Rare

Flu: Sudden onset; can be severe

Pneumonia: May occur depending on type

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• MUSCLE ACHES

Cold: Mild

Flu: Usual; often severe

Pneumonia: May occur depending on type

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• FATIGUE AND WEAKNESS

Cold: Mild

Flu: Can last two or more weeks

Pneumonia: May occur depending on type

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• EXTREME EXHAUSTION

Cold: Never

Flu: Sudden onset; can be severe

Pneumonia: May occur depending on type

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• RUNNY, STUFFY NOSE

Cold: Common

Flu: Sometimes

Pneumonia: Not typical

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• SNEEZING

Cold: Common

Flu: Sometimes

Pneumonia: Not typical

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• SORE THROAT

Cold: Common

Flu: Sometimes

Pneumonia: Not typical

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• CHEST DISCOMFORT, COUGH

Cold: Mild to moderate; hacking cough

Flu: Common; can become severe

Pneumonia: Severe cough; can include stabbing chest pain

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• COMPLICATIONS

Cold: Sinus and ear infections

Flu: Bronchitis, pneumonia; can be life-threatening

Pneumonia: Widespread infections of other organs; can be life-threatening especially in elderly and debilitated patients

Sources: American Lung Association, Centers for Disease Control and Prevention