This is a direct quote from Potter's Fundamentals of Nursing:
"Course of Infection by Stage"
Interval between entrance of pathogen into body and appearance of first symptoms (e.g., chickenpox, 2-3 weeks; common cold, 1-2 days; influenza, 1-3 days; mumps, 15-18 days)
Interval from onset of nonspecific signs and symptoms (malaise, low-grade fever, fatigue) to more specific symptoms (During this time, microorganisms grow and multiply, and client may be more capable of spreading disease to others)
Interval when client manifests signs and symptoms specific to type of infection (e.g., common cold manifested by sore throat, sinus congestion, rhinitis; mumps manifested by earache, high fever, parotid and salivary gland swelling)
Interval when acute sypmtoms of infection disappear (Length of recovery depends on severity of infection and client's general state of health; recovery may take several days to months)
By understanding the chain of infection, the nurse intervenes to prevent infections from developing. When the client acquires an infection, the nurse observes signs and symptoms of infection and takes appropriate actions to prevent its spread. Infections follow a progressive course (that is the stages above that I have typed). The severity of the client's illness depends on the extent of the infection, the pathogenicity of the microorganism, and the susceptibility of the host.