Disease-causing microorganisms are called as:
Pathogens
A pathogen or infectious agent is a biological agent that causes disease or illness to its host. This term is most often used for agents that disrupt the normal physiology of animal or plant. Pathogens can be a bacterium, virus, or other microorganisms that can cause disease.
The term disease causative agent usually refers to the biological pathogen that causes a disease, such as a virus, parasite, fungus, or bacterium. Technically, the term can also refer to a toxin or toxic chemical that causes illness.
A carrier is a person infected with an infectious disease agent but displays no symptoms. A vector is an organism that does not cause disease itself but which spreads infection by conveying pathogens from one host to another.