With 17th of Tammuz coming up in a few days, and 9th Av three weeks later, several people have suggested that fasting might be a danger, as it could weaken one’s resistance to the COVID virus. What is the Rav’s ruling in this case for an otherwise healthy person, and for a person with comorbidities? Also, is there a difference between 17th of Tammuz and 9th of Av? Thanking you in advance,
Yaakov
yaacov wolf 
First ask your doctor. If doctor says not to fast, don’t fast on either.
17 Tamuz is just a Minhag. If you’re not sure or if you have issues, don’t fast.
Tisha Be’av is more stringent, but if there’s any health risk, you cannot fast.
Yom Kippur is a bit more complicated, as it is from the Torah, and we can address that as it nears. An example on Yom Kippur is that even if you need to break the fast for health reasons, I’d encourage you to take like a bottle of Gatorade and sip it from a shot glass, taking a full shot glass every 4 minutes. We will discuss this then.