How do people get sick? I don't understand why we get fevers, headaches, and coughs when we're sick.

Dear Min Jung:

Every day, our bodies come in contact with viruses and bacteria that are too small to be seen, but could make us sick. Our skin keeps out most of these viruses and bacteria, but some get into our bodies through our lungs (in the air we breathe) or our stomach (through the food and drink we digest). Still others enter our body when our skin is cut or scraped.

Luckily, most harmful viruses and bacteria never get the chance to make us sick. Our bodies immediately attack and destroy these invaders by using special white blood cells and chemicals in our bloodstream. These special cells work by surrounding and digesting some of the invaders. The chemicals, called antibodies, quickly kill or neutralize the others.

Unfortunately, sometimes there are too many viruses and bacteria for our bodies to handle at once. The viruses and bacteria that are not killed quickly can give off poisonous substances called toxins. These toxins can travel to all parts of our bodies and damage some of our body cells. That is why we get a headache, fever, or muscle aches when we get sick.

It's also possible for the viruses or bacteria or their toxins to attack only certain areas. If they enter the nose and throat area, we will have a runny nose and sore throat. If they attack the lining of the stomach or intestines, vomiting or diarrhea may occur. Infection in the lungs or respiratory tract can give us a cough.

If our bodies are strong, however, we can expect to feel better in about three days even though we may not be back to perfect health. That's usually how long it takes to start winning the battle against most infections such as colds.