Even though visiting someone who is ill is common sense, it none the less is a Divine command. Visitations bring up the morale of the ailing and give him/her more of a desire to recover. The mitzvah bikur cholim (visiting the sick) is divided into two parts: concern for the physical well being of the ill, and praying for the welfare of that person. Fulfillment of this mitzvah can include easing a person's recovering by bringing items such as food and reading materials. If visiting in person is not possible, even calling on the phone can be a fulfillment of this mitzvah.
Genesis, Isaiah, "And you shall follow in His ways."
The Rambam (Maimonides), one of the greatest legal and philosophic masters in Jewish history in addition to be a world-renowned physician, discusses the practice of visiting the sick in his encyclopedic work of Jewish law, the Mishnah Torah. The Rambam groups the mitzvah of visiting the sick together with many other acts of chesed (kindness) such as aiding travelers, comforting mourners, burying the dead, helping people get married. The fact that we are dealing with a mitzvah means that in Judaism there is a spiritual context for chesed. In the case if Bikur Cholim, we not only care about the health of the patient, but we want G-d to know that we care. In addition, we are taught that G-d is present with us at the sick bed.

An important component of the mitzvah of Bikur Cholim is to pray for the recovery of the sick person. This is typically done when praying the Shmoneh Esray's eighth prayer, being the one for refuah (healing). You can also make a "Mi Sheberach" for cholim, which is typically done during Shabbos morning prayers. The name of the sick person is given to the gabbai, who recites his/her name. Both for the mishaberach and for personal prayer, the Hebrew name of the person needing a refuah is given using his or her mother's name rather than that of the father. If the Hebrew name is not known, then their English name may be used. Another powerful way to pray for a refuah shlaima (complete recovery) is to recite chapters from Tehilim (Psalms).
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