Microorganisms capable of causing disease—pathogens—usually enter our bodies through the mouth, eyes, nose, or urogenital openings, or through wounds or bites that breach the skin barrier. Organisms can spread—or be transmitted—by several routes.

What does a pathogen do when it enters the body?

Pathogens are different and can cause disease upon entering the body. All a pathogen needs to thrive and survive is a host. Once the pathogen sets itself up in a host's body, it manages to avoid the body's immune responses and uses the body's resources to replicate before exiting and spreading to a new host.

How does the body respond when a pathogen first enters the body?

Once infected cells have sensed an invading pathogen, they secrete molecules called cytokines and chemokines. Cytokines such as interferons are molecules that signal neighbouring cells and induce an antiviral state in them. These cells are then primed to resist an infection with the invading virus.