Humans contract bacterial infections such as tuberculosis or syphilis. what is this ecological relationship called?

The answer is parasitism. Parasitism is a relationship in which one species benefits and the other does not benefit. Predation is similar to parasitism in that both types of relationship benefit one of the interacting species while harming the other. Parasites are generally smaller than their host.

Parasitism

Parasitism can be defined as a relationship between species in which one organism (parasite) lives on or in another organism which is the host, causing it some harm without immediately killing it. Thus, parasitism is a kinds of close, symbiosis and persistent biological interactions.