Disease production by pathogens

The first stage of disease production by pathogens is infection, or entry of the pathogen into the body. Nearly all pathogens must gain entry into the body before they can begin to replicate or spread. A few pathogens can exist on the skin (e.g. viruses causing warts) or in the gut (e.g. bacteria causing cholera) without technically entering the body. However, infection is not made easy for pathogens because the body has many physical and chemical barriers to try to prevent pathogens entering the body.