What causes infectious disease in humans? certain bacteria enter and die within the human body. certain viruses change and reproduce outside the body. certain pathogens enter and multiply within the human body. microorganisms within the body develop mutations.

Certain pathogens enter and multiply within the human body.

Explanation:

Infection or invasion of a pathogen inside the body of the human host does not necessary causes the disease. Instead the multiplication and colonization of these pathogens inside the body of the human host is responsible for disease. This is because of the fact that the pathogen's cells multiply and invade the healthy cells of the human host. This process weaken up the host immune system. This process leads to the occurrence of signs and symptoms of the disease.