A-
HUHS Is Your Friend
Every student is required to pay a nominal health fee each year. However, that covers everything from doctor's appointments when sick, mental health counseling, vaccinations, dental care, and care when you've had one too many on a Saturday night, all at an massive infirmary right on campus. While the infirmary staff can be slow and unresponsive when the place is busy, the doctors and professionals are excellent, what you would expect from a school affiliated with at least a dozen hospitals.
As for safety on campus, there are constant security patrols, most gates close at night, and very few on-campus incidents are reported (the occasional off-campus mugging is the only news item). There are some more unsavory folks occasionally hanging out by the subway stop in Harvard Square, but the area is generally well lit and there are enough people milling about at all hours to make you feel consistently safe.