Given the complexities of host defenses and microbial virulence, it is not surprising that the factors determining outcome between these two living entities are also complicated. Basically, outcome depends on the state of the host's health, the virulence of the pathogen, and whether the host can clear the pathogen before infection and disease cause irreparable harm or death (Figure 3-11).
The time for a disease or infection to develop also depends on host and microbial factors. Infectious processes that develop quickly are referred to as acute infections, and those that develop and progress slowly, sometimes over a period of years, are known as chronic infections. Some pathogens, particularly certain viruses, can be clinically silent inside the body without any noticeable effect on the host before suddenly causing a severe and acute infection. During the silent phase, the infection is said to be latent. Again, depending on host and microbial factors, acute, chronic, or latent infections can result in any of the outcomes detailed in Figure 3-11.
Medical intervention can help the host fight the infection but usually is not instituted until after the host is aware that an infectious process is underway. The clues that an infection is occurring are known as the signs and symptoms of disease and result from host responses (e.g., inflammatory and immune responses) to the action of microbial virulence factors (Box 3-10). The signs and symptoms reflect the stages of infection. In turn, the stages of infection generally reflect the stages in host-microorganism interactions (Figure 3-12).
Figure 3-12 Host-microorganism interactions and stages of infection or disease.
Encounter and entry
Colonization and entry
Invasion and dissemination
Pathogen encounters and colonizes host surface
Pathogen multiplies and breaches host surface defenses
Pathogen invades deeper tissues and disseminates, encounters inflammatory and immune responses
No signs or symptoms
First signs and symptoms, pathogen may be highly communicable
Peak of characteristic signs and symptoms of infection or disease
Stage of decline
Condition of host deteriorates possibly to death or signs and symptoms begin to subside as host condition improves
Full recovery of surviving host or chronic infection develops, or death
BOX 3-10 Signs and Symptoms of Infection and Infectious Diseases
General or localized aches and pains
Swollen lymph nodes Rashes
Redness and swelling Cough and sneezes
Congestion of nasal and anus passages Sore throat Nausea and vomiting Diarrhea
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