When our forefather Yaakov became sick and bed-ridden (choleh she-nafal le-mishkav), he became the first such person mentioned in the Torah. What are the various laws dealing with such a choleh, and when is he exempt from certain mitzvot because of illness and its accompanying weakness?
A choleh is exempt from the mitzva of living in a sukkah, as are his caretakers. This is true not only for someone who is dangerously ill, but even for someone who merely has a headache or sore eyes. (This exemption is specific to the mitzva of sukkah, and one should not extrapolate from it to other mitzvot.) A choleh is also exempt from traveling to Jerusalem for the three major festivals of Pesach, Shavuot, and Sukkot (aliyah le-regel). Those who can travel are obligated, while those who cannot are exempt. There are mitzvot from which a choleh is exempt because it is assumed he will not be able to summon the requisite levels of concentration, such as the mitzva of tefillin. Additionally, a person wearing tefillin must be able to control his bodily functions (guf naki). Somebody sick is likely to be unable to do so.
Normally, people are required to stand out of respect for a king or prince, an elderly person, or a talmid chacham (Torah scholar). Sick people are exempt from doing so. This is either because they are understandably preoccupied with their pain, and thus cannot show the proper respect, or because when sick people stand, it is not seen as showing honor. The difference between these two reasons comes into play in a case where a sick person chooses to stand. If the reason that sick people are exempt is because they are preoccupied with their pain, one choosing to stand would indicate he has overcome this difficulty. However, if the reason is that the rising of someone in a weakened state does not show honor, then perhaps he should be asked to sit.
The Talmud (Moed Katan 27b) states that if a sick person stands up for a king, we do not tell him to sit. Some understand this to mean that a sick person may stand up if he wishes. This fits with the behavior of our forefather Yaakov, who exerted himself and sat up in bed (Bereishit 47:31).
However, others explain that the reason we do not tell a sick person to sit down is that it might sound as if we are saying, “Sit in your illness,” meaning “Stay sick,” which would be insulting. According to this approach, the Talmud does not permit a sick person to stand. As we said above, it is even possible that such standing does not show respect. If this is the case, why did Yaakov act as he did? A close reading of the verse indicates that Yaakov did not stand, but rather sat up in bed. Out of respect for the king he sat up, but went no further than that.