Regarding fasting on all fasts with the exception of Yom Kippur, the halacha is that someone who is not well but there is no danger to their life, is exempt from fasting, and according to many opinions this includes Tisha Be’Av as well. Such a person is one who feels generally not well, is lying in bed and would not go to work, is feeling dizzy, faint, or weak. It should be noted that some of these situations may sometimes halachically even be considered a sakana as well. A person who is not well, even if there is no danger to their life, does not have to fast. This also includes a person who is generally weak, for example a senior citizen, or someone who recently had surgery, even though there is no danger to their life. All of those people should not fast on any of these fasts, and according to many poskim this includes Tisha Be’Av too. Yom Kippur, of course, has totally different rules.