Every public place has warning signs about not spitting, the main reason is to prevent (), because sputum contains bacteria and so on. A. digestion.

The original parasitic position, transmission route and main cases of four infectious disease pathogens in human body are shown as follows: Respiratory infectious diseases are mainly transmitted by droplets and air, so measures such as not spitting, keeping air circulation in houses and public places and wearing masks are effective in preventing respiratory infectious diseases.

Therefore, choose: B.