A pathogen is a disease-provoking microorganism, such as a bacterium or a virus, which enters a host and disrupts its normal functions, sometimes causing the death of the host body. A pathogen can penetrate a host through three main pathways: through the mouth (oral), mucous membrane (nose, eyes, etc), and through an open would.
The most common pathogens are bacteria, viruses, protozoans (parasites), fungi, and prion. Pathogens can be transmitted from person to person in a number of ways. The influenza virus is transmitted from person to person through the air, through sneezing and coughing. Escherichia coli is readily transmitted through water, food, and blood, but is not readily transmitted via air or the bite of an insect. Nevertheless, the body contains many natural order of defense against some of these pathogens; this natural defense is the immune system, which is composed of white blood cells.