Answer:
It is known as a parasitic relationship.
Explanation:
Parasitic relationships are exactly what you described. We see these sorts of relationships in nature in worms for instance.
When a worm attaches to a host, it oftentimes leeches off the host's blood for nutrients and the like. In this manner, the worm benefits while the host gets weaker and loses nutrition.
Other examples include ticks and leeches.
