The International Classification of Diseases (ICD) is used in clinical settings to classify diseases and track morbidity (the number of cases of a disease) and mortality (the number of deaths due to a disease) rates.

Classifications of Disease

The five phases of disease are:
- Incubation
- Prodromal
- Illness
- Decline
- Convalescence

Phases of Disease

Signs of disease are objective and measurable, and can be directly observed by a clinician.

Signs of Disease

Symptoms of disease are subjective. They are felt  by the patient, but they cannot be clinically confirmed or objectively measured by a clinician.

Symptoms of Disease

- Skin & Eye
- Respiratory System 
- Urogenital System 
- Digestive System 
- Circulatory System
- Lymphatic System
- Nervous System
- Immune System

Body Systems Affected by Disease

Psychosomatic disorders are physical illnesses that are thought to arise from emotional or mental stressors, rather than direct physical causes. These conditions demonstrate how psychological factors can significantly influence physical health, often manifesting as pain, fatigue, or other somatic symptoms without an identifiable medical explanation.

Psychosomatic Disorders

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

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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.

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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

Disease

- Opportunistic pathogen
- Primary pathogen

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