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

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

Phases of Disease

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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

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.

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