In biology, there are animals that have different relationships with each other. Parasitism is when an organism lives off of another animal in order to survive. They most likely get their nutrients and food from this organism. One example of this is a leech.
A leech may live on an animal in order to suck its blood because leeches live off of blood. Another example of parasitism is when lichen attaches itself to trees. It takes the nutrients from the trees, but this causes the trees to die down.
However, the lichen must be attached to a tree in order to survive. The problem with this is that usually the other organism doesn’t like this organism attaching itself to them because it causes them problems.