Rambam, Laws of Mourning 14:4-5, (cf. Shulchan Aruch, Yoreh De'ah, 235-238)

Visiting the sick is a mitzvah incumbent upon all and even the great ones visit the small ones. We visit many times a day and the one who visits more is more praiseworthy, provided they don’t bother the sick ones with their visits. And anyone who visits a sick person, it’s as if he lifted part of his illness and made it lighter. And anyone who doesn’t visit, it’s as if he’s a spiller of blood. We don’t visit the sick person, except from the third day of the illness and onward. But if the illness overtook him suddenly and became severe, we visit him right away. And we don’t visit the sick person either in the first three hours of the day or in the last three hours of the day because that is when they are busy with the needs of the sick person. And we don’t visit those who are sick with intestinal diseases or with eye diseases or with headaches because the visiting is difficult for the sick ones. [AJWS translation]