Is there a causal relationship between smoking and pneumonia? What research methods can be used?

There is no necessary causal relationship between new pneumonia and smoking. However, smoking will reduce people's resistance. If you pick up the cigarette butts of people infected with COVID-19 virus, you may be in danger of cross infection. Smoking can cause many kinds of harm to human body. As far as the respiratory system is concerned, smoking will weaken the immune function of the respiratory tract and reduce the resistance to viruses. At present, in the absence of specific drugs, the elimination of novel coronavirus mainly depends on the human immune system. Therefore, in theory, long-term heavy smoking is not conducive to prevention in COVID-19.