How can infectious disease be caused?

Answer:
Infectious diseases are caused by the invasion of a microorganism to the body in sufficient concentration /number to produce disease.
The severity of the disease is depend up on the virulence of the organism and the compatibility of the host.
First answer by Ramatneethi. Last edit by Ramatneethi. Contributor trust: 2 [recommend contributor recommended]. Question popularity: 1 [recommend question].