At the burning bush Moses was asked to take off his shoes because he was standing on holy ground.
 Now Moses was keeping the flock of his father-in-law, Jethro, the priest of Midian, and he led his flock to the west side of the wilderness and came to Horeb, the mountain of God.  And the angel of the LORD appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed.  And Moses said, “I will turn aside to see this great sight, why the bush is not burned.”  When the LORD saw that he turned aside to see, God called to him out of the bush, “Moses, Moses!” And he said, “Here I am.”  Then he said, “Do not come near; take your sandals off your feet, for the place on which you are standing is holy ground.” (Exodus 3:1-5, ESV)
Joshua, the person appointed to take over leadership of the people of Israel from Moses also had his “holy ground” experience:
 When Joshua was by Jericho, he lifted up his eyes and looked, and behold, a man was standing before him with his drawn sword in his hand. And Joshua went to him and said to him, “Are you for us, or for our adversaries?”  And he said, “No; but I am the commander of the army of the LORD. Now I have come.” And Joshua fell on his face to the earth and worshiped and said to him, “What does my lord say to his servant?”  And the commander of the LORD’s army said to Joshua, “Take off your sandals from your feet, for the place where you are standing is holy.” And Joshua did so. (Joshua 5:13-15, ESV)
This, of course, raises the question, “Do we still have holy ground today.” As it turns out, indeed we do. One of the major emphases of Martin Luther’s theology was that God uses suffering to strengthen our faith. Instead of looking at troubles as an indication that there is something wrong with our faith or that God is punishing us, we should receive suffering as God’s way of strengthening our faith by withdrawing our affections from this earth and focusing on the eternal realities that are ours through faith in Jesus Christ.
I Peter supports this kind of thinking, starting with this passage from the first chapter:
In this you rejoice, though now for a little while, if necessary, you have been grieved by various trials,  so that the tested genuineness of your faith—more precious than gold that perishes though it is tested by fire—may be found to result in praise and glory and honor at the revelation of Jesus Christ. (I Peter 1:6-7, ESV)
Our faith is more precious than gold. Just as gold is purified by fire, our faith is purified by various grievous trials.
And then there is this passage from chapter two:
For this is a gracious thing, when, mindful of God, one endures sorrows while suffering unjustly. (I Peter 2:19, ESV)
And James chimes in with:
Count it all joy, my brothers, when you meet trials of various kinds,  for you know that the testing of your faith produces steadfastness.  And let steadfastness have its full effect, that you may be perfect and complete, lacking in nothing. (James 1:2-4, ESV)
Here are some quotes from Luther to show his perspective:
“When faith begins, God does not forsake it; He lays the holy cross on our backs to strengthen us and to make faith powerful in us.” (Lutheran Study Bible, p. 789)
Suffering is, “a holy possession whereby the Holy Spirit not only sanctifies his people, but also blesses them.” (From Luther’s 1539 treatise On Councils and the Church)
So since suffering makes us holy by strengthening our faith, wherever a believer is suffering is holy ground. I think this makes a big difference when we go to visit those who are sick. We are entering holy ground because God is there strengthening their faith through the suffering.
One final thought from Luther:
Where suffering and the cross are found, there the Gospel can show and exercise its power. It is a Word of life. Therefore it must exercise all its power in death. In the absence of dying and death it can do nothing, and no one can become aware that it has such power and is stronger than sin and death. (Luthers Works, American Edition, vol. 30, pages 126-127)
For more about Luther’s view of suffering see:
Kenneth J. Woo (http://sites.duke.edu/kennethwoo/), Concordia Theological Quarterly, Volume 77, Number 3-4, July/October, 2013, pp. 307-325.