When to begin Davening for a sick person-How sick must they be?
When someone has become ill, one is not to spread the word to the public until several days pass. The reason for this is because when one spreads the word that an individual is sick, and people view him in this status, his Mazal becomes diminished. One may, however, spread the word to family and friends even on the first day. If the death is imminent, then one may spread the news even on that day. Based on the above, it is ruled that when a person falls ill, family and friends may visit the ill person right away, while other individuals are only to visit after the passing of three days from when he fell ill. If, however, death is imminent, then everyone may visit immediately. [Based on this, one can learn that one should not prematurely spread negative medical news about an individual or a relative prior to confirmation of their illness and as to its severity. Even if one has good intents in doing so in order so people pray for their wellness, or to get a good diagnosis, it has the negative consequence of spreading the news that they are sick and worsening their Mazal. Likewise, based on this one should only add a person’s name to Mi Shebeirach for the sick or for Tehillim recital, if their illness has been confirmed and diagnosed. By sudden illness it should only be spread after three days unless death is imminent r”l, as stated above. Of course, the intent of spread is to tell the public, however family and close friends may be told right away when there is medical concern. Likewise, based on the above we can understand that once a person has been healed from his illness his name should be removed from the Mi Shebeirach or Tehillim lists for the sake in order not to diminish his Mazal.]
 Taz 335:1; Nedarim 40a
 Bach, brought in Taz 335:1; Nedarim ibid
 Michaber Y.D. 335:1; Yerushalmi Peiah end of Chapter 3; Miseches Semachos; See Likkutei Sichos 5:78
The reason: In order not to spread the fact that he is sick and ruin his Mazal. [Taz 335:1; Bach 335]