What is one way that some bacteria are to the human body? how do some bacteria harm the body?

Some bacteria can help you nutritiously, and some can get you sick, if you don't wash your hands.

Smoking damages the respiratory system by inhalation of a foreign substance, that is absorbed and residue is left behind on our lungs sponge like substance. the gas that is inhaled also increase your blood pressure and dilates your blood vessels. also the process of inhaling and exhaling the smoke it causes irritation to the throat and mouth. smoking can cause lung cancer and other respiratory and circulatory diseases. 

The answer is either 3 or 4