Surgical masks worn by medical personnel are not to protect them from the patient but to protect the patient from them. The medical personnel would give a pathogen to the patient. Masks, except N95s, do not protect the healthcare provider from the patient. There is about a week incubation time from exposure to symptoms, so medical personnel could have been infected and not know it. Also, most healthcare facilities lack adequate protective gear.
It is difficult to determine whether or not an individual's immune system is "weak" or not. As we age, our immune system does not handle pathogens as well but still mounts an immune response. In fact, a "weakened" immune system might even over-react in an attempt to make the immune response even more potent. Sepsis is caused by the immune system reaction. Over a million people in the U.S. will become septic with a quarter of those people dying from it. Beware when someone tells you that you need to "boost your immune response". Then again, we all need a healthy immune system.