Your real weak points to infections are your mucous membranes—the surface that lines your windpipe and lungs, eyelids, mouth, and nose, your stomach and intestines, your reproductive tracts and bladder. It is hard to give their total surface area since numbers vary so much from person to person, but on average there are about 200 square yards (meters) of mucous membranes in a healthy adult (about the same as a tennis court), most of them being your lungs and your digestive tract.
