Clinical Infectious Disease Process

There are four stages of the disease process

1. Incubation Period 

2. Prodromal Stage 

3. Invasion Period 

4. Covalescence 

First stage : Incubation Period 

This is the interval between exposure to an infectious agent and the appearance of the first symptoms. 

• The microorganism has entered the body, undergone colonisation and begun multiplying. 
• However, symptoms will not have started yet due to insufficient numbers. 
• Can last anywhere from hours to days. 
• For example, the flu takes 1-2 days to show symptoms whereas COVID-19 can take from 2-14 days. 

Second stage : Prodromal Stage 

This is the appearance of initial symptoms that are often mild,usually having feelings of tiredness and discomfort. 

• The pathogen begins to multiply, creating general signs and symptoms of illness. 

• Typically a result from the activation of the immune system, such as fever, pain, soreness, swelling, or inflammation. 

Third stage : Invasion Period 

This is when the pathogen multiplies rapidly, invading further, affecting tissues at site of initial colonisation and other areas. 

• Immune and inflammatory responses are being triggered 

• Symptoms develop specifically related to the pathogen and symptoms related to the protective inflammatory response

• Signs and symptoms of disease are the most obvious and severe at this stage 

Fourth stage : Convalescence 

Individual's immune and inflammatory systems have in general successfully removed infectious agent. 

• The symptoms decline during this stage. 

• However, the disease may be fatal or enter a latency phase, meaning the symptoms have resolved but may reactivate at another time