An encounter with a potential pathogen is known as exposure or contact. The food we eat and the objects we touch are ways that we can be exposed to potential pathogens. For a pathogen to cause disease, it needs to be able to gain access into host tissue. An anatomic site through which pathogens can pass into host tissue is called a portal of entry. 

Exposure

Following the initial exposure, the pathogen adheres at the portal of entry. Adhesion refers to the capability of pathogens to attach cells of the body using adhesion factors. Different pathogens use various mechanisms to adhere to the cells of the body.

Adhesion

Once adhesion is successful, invasion can proceed. Invasion involves the wide spread of a pathogen throughout local tissues or the body. Pathogens may produce exoenzymes or toxins, which serve as virulence factors that allow them to colonize and damage host tissues as they spread deeper into the body. 

Invasion

Following invasion, successful multiplication of the pathogen leads to infection. Infection is the final stage of pathogenesis.

Infection 

There are four stages of pathogenesis. The pathogen must be able to gain entry to the host, travel to the location where it can establish an infection, evade or overcome the host’s immune response, and cause disease to the host.
- exposure (contact)
- adhesion (colonization)
- invasion
- infection.

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Pathogenic bacteria are bacteria that can cause infectious disease.

Pathogenic Bacteria

https://openstax.org/details/books/microbiology/

Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

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Virulence is the degree to which an organism is pathogenic. Highly virulent pathogens will almost always lead to a disease. Less virulent pathogens may cause an initial infection, but they do not always cause severe illness. Some individuals might even be asymptomatic. Two important indicators of virulence are:
- the median infectious dose (ID50) 
- the median lethal dose (LD50)

Virulence

Stages of Pathogenesis

A disease is any condition in which the normal structure or function of the body is damaged or impaired.

Disease

- Opportunistic pathogen
- Primary pathogen

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