The incubation period occurs after the initial entry of the pathogen into the host. During this time, the pathogen begins multiplying in the host. However, there are insufficient numbers of pathogen particles to cause signs and symptoms of disease. Incubation periods can vary from a day or two in acute disease to months or years in chronic disease.

Incubation Phase of Disease

The prodromal period occurs after the incubation period. During this phase, the pathogen continues to multiply and the host begins to experience general signs and symptoms of illness such as fever, pain, soreness, swelling, or inflammation. Signs and symptoms are too general at this stage to indicate a particular disease.

Prodromal Phase of Disease

The illness phase occurs when the signs and symptoms of disease are most obvious and severe.

Illness Phase of Disease

The period of decline occurs when the number of pathogen particles begins to decrease, and the signs and symptoms of illness begin to decline. However, during the decline period, the immune system is weaker, and patients may become susceptible to developing secondary infections.

Decline Phase of Disease

The final phase of disease is the convalescence period. During this stage, the number of pathogens is very low and the patient generally returns to normal functions. Some diseases may inflict permanent damage that the body cannot fully repair.

Convalescence Phase of Disease

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

Austin Community College

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

Disease

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.

OpenStax Microbiology Textbook

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

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.

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