Correct answer: Microorganism
The microorganism that causes the disease are known as pathogens and the disease that is caused by them are called an infectious disease.
The germ theory is the scientific theory of disease. It states that most of the diseases are caused by microorganisms. These microorganisms are too small that they can not be seen by the naked eye and are known to invade humans and other organisms.
Example: Bacteria, Fungi, Protists, and viruses.