A primary pathogen can cause disease in a host regardless of the host’s microbiota or immune system.

Primary Pathogen

An opportunistic pathogen can only cause disease in situations that compromise the host’s defenses, such as skin, immune system, or normal microbiota.

Opportunistic Pathogens

- Opportunistic pathogen
- Primary pathogen

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Pathogenic bacteria are bacteria that can cause infectious disease.

Pathogenic Bacteria

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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

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.

Stages of Pathogenesis

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

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