What do you call organisms that rely on a host for survival without contributing to that host's well-being?

The correct term for organisms that depend on a host for survival while not benefiting that host is "parasites." Parasites exist in a relationship where they derive sustenance at the host's expense, often leading to harm or negative effects on the host, such as disease or depletion of resources. They can be classified into different categories, including protozoa, helminths (worms), and ectoparasites like fleas or ticks.

In contrast, viruses, while they do require a host to reproduce, are fundamentally different in that they hijack the host's cellular machinery to reproduce and can cause illness in the host, but are not classified as living organisms by many scientists. Bacteria can be beneficial (like those in the gut) or pathogenic (causing disease), and fungi can also have both beneficial and harmful effects. However, parasites are specifically characterized by their exploitative relationship with hosts, which is why this term accurately reflects the organisms described in your question.