The obligation to fast on Tisha B’Ab does not apply to ill patients, even those whose condition is not life-threatening. For example, if a person has a fever on Tisha B’Ab, he is exempt from fasting. This exemption also applies to those who are frail and to the elderly. (Hacham Ben Sion Abba Shaul ruled that people aged 80 and above are exempt from fasting on Tisha B’Ab.) Of course, ill or frail individuals should consult with a physician and with a Rabbi to determine their status vis-à-vis the Tisha B’Ab fast.
The Hid”a (Rav Haim Yosef David Azulai, 1724-1807) ruled that those who are exempt from fasting on Tisha B’Ab may eat normally, as they wish. Tisha B’Ab differs in this respect from Yom Kippur, when ill patients who must eat are required to limit their eating and drinking to specific amounts within certain time-frames. On Tisha B’Ab, those who are exempt are permitted to eat and drink freely according to their needs.
When Tisha B’Ab falls on Mosa’eh Shabbat, as it does this year (5772), those who are exempt from fasting must first recite Habdala. As Halacha forbids eating after Shabbat before Habdala, those who are allowed to eat on Tisha B’Ab must ensure to first recite Habdala after Shabbat. They should recite the Beracha of “Ha’gefen” over a cup of wine or grape juice, followed by the Beracha of “Ha’mabdil Ben Kodesh Le’hol,” and then drink a “Melo Lugmav” (cheek-full). Afterward, they are free to eat and drink as they wish.
It should be noted that those who eat bread on Tisha B’Ab do not add the paragraph of “Nahem” in Birkat Ha’mazon.