Not all pathogens are invasive. Some cause disease by multiplying on the bodies external or internal surfaces. But many pathogens are invasive, this means that they penetrate the surface and gain access to the bloodstream. Invasion of the bloodstream is always dangerous, because it allows microorganisms to spread throughout the body. Normally sterile organs and structures may become infected.
Infection stimulates specific and non-specific host immune responses.
We will cover the basic features of the immune system in later lectures. To be successful, a pathogen must thwart the immune system. The disease process involves damage of tissues. Different microorganisms damage host tissues in different ways. Some of the most common ways that pathogens harm tissues are listed below.