Smoking is bad because, it can cause lung cancer, and other infectious diseases. Which can be deadly, if not treated.
Here are some of tings that can happen inside of the lung's if you smoke:
1) Cancer
2) Asthma
And many more.
Smoking doesn't just affect the lung's, smoking alos effect the mouth and teeth.
Smoking can cause teeth to turn yellow, and smokers often have bad breath from smoking. Smoking can alos cause cancer inside of the mouth, and even on the lips. Which can be deadly if not treated.