The question here is where to draw the line between commensalism and parasitism. At the extremes a distinction is obvious. Some parasites can be extremely damaging and cause death, while some commensals derive only shelter and protection from the host without using any materials or placing any limitations on the host. A distinction becomes more difficult when one considers parasites that cause very little damage to the host or commensals that obtain substantial materials from the host. The student has a valid point when considering this “middle ground” where a distinction is difficult.