A. "Wash your hands frequently and pay attention to personal hygiene" can prevent pathogens on your hands from entering the human body through the digestive tract, that is, cut off the route of transmission;

B, "SARS" patients are the source of infection, it is forbidden to visit "SARS" patients to prevent pathogens from being transmitted to healthy people through the respiratory tract, which is to cut off the route of transmission;

C, wearing a mask in a crowded place can prevent pathogens from spreading through breathing, and it also belongs to cutting off the route of transmission;

D, SARS patients can spread pathogens, belonging to the source of infection, and isolation treatment belongs to controlling the source of infection.

So choose: D.