Q. Is it true that you are not allowed to visit a sick person during the first three days of his illness? Why?

A. Shulchan Aruch (Y.D. 335:1) rules that close family members and friends visit the sick immediately, while others wait until three days have passed. Taz (ibid. 1) quotes Bach who explains that the reason is not to worsen the chole’s mazal. We don’t want to label him yet as a sick person, since he may just get well rapidly. The Talmud (Nedarim 40a) relates that Rabba would on the first day of illness refrain from telling anyone, so his mazal should not worsen. After three days have passed there is already an established chazaka status of being a choleh and should be visited by all.

However, if stricken with a severe life threatening illness and there is concern that he may pass away shortly, one can visit immediately when visiting is medically allowed. (Shulchan Aruch ibid.) The above also does not apply to people living in the same house. (See Rambam H. Avel 14: 5)

Horav Shlomo Miller Shlit”a stressed that even on the first three days there is no real visiting prohibition, it is more analogous to being a custom and if the sick will benefit from the visit it is permitted.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a