Nedarim 39 - Visiting the sick
The hint to the mitzvah of visiting the sick comes from the Korah's rebellion. Moses said, " If these people die like all men, and are visited ..." - and from here we see that when normally people get sick before they die, it is expected that they should be visited.
If one vows against a fellow, he may still visit him if the fellow gets sick, but he should stand and not sit. What are we talking about? If the visitor is prohibited to benefit the sick one, then the visitor should be allowed even to sit: he is doing a mitzvah. And if the sick is prohibited to give benefit to the visitor, then no visits should be allowed, since the visitor inevitably derives some benefit by entering the house, which is more protected than being in the street!?
Really, possessions of the visitor are prohibited to the sick. So the visitor should be able to enter and even sit with the sick person. However, the visitor may sit more than necessary, going beyond the bounds of the mitzvah and providing the sick person with the extra benefit, for which one could even claim a reward. By doing this for free, the visitor would be violating the vow. Therefore, the Sages allowed him to stand - since he will leave on time - but not to sit.
Art: The Sick Room by Emma Brownlow