In terms of fasting on Shiva Asar BeTamuz, there is more for leniency this year for anyone feeling weak or who has difficulty fasting in general (see Yalkut Yosef, Moadim, pp. 531-532). Pregnant and nursing women should not fast, even if they are normally stringent (see Rema, Orach Chaim 550:1, MB 3-4). Tisha Be’av has a stricter standard for pregnant and nursing women, and the presumption for most healthy people is that they should fast unless they are bedridden or in such severe discomfort that they are unable to function normally (חולה שאין בו סכנה). However, in connection with both fasts including Tisha B’av this year, anyone who is immuno-compromised or above 60 should not fast in my opinion unless approved by a doctor. If the doctor feels that there is a risk for someone in that category who is otherwise healthy, or for anyone else who is otherwise healthy, that person should eat normally on Shiva Asar BeTamuz, and פחות פחות מכשיעור on Tisha Be’av (i.e., one ounce shot glasses of water and/or chopped up food in intervals of 9 minutes) unless the doctor feels that they need to eat regularly (see Biur Halacha 554, ד”ה דמקום חולי regarding the observance of Tisha Beav during the cholera epidemic).
[undated, July 7, 2020, or prior]