Stages of Disease: Incubation PeriodTime between infection and onset of signs and/or symptoms

Factors affecting the incubation period:Virulence of the pathogenInfective doseHealth of the patient's immune systemGeneration time of the pathogenSite of infectionTherefore, incubation periods differ depending on the pathogen involved

Stages of Infectious Disease:Prodromal PeriodShort time of generalized, mild symptoms preceding illnessSymptoms often include malaise and muscle achesNot all infectious diseases have a prodromal period

Stages of Infectious Disease:IllnessMost severe stage of an infectious diseaseSigns and symptoms are most evident at this timePatient's immune system has not yet fully respondedPathogen is harming the body and causing diseaseA physician typically sees the patient during this stage

Stages of Infectious Disease:DeclineBody gradually returns to a normal state of healthImmune response and/or medical treatment begin to affect the growth and survival of the pathogenSigns and symptoms subsideThe immune response and antibody titers normally peak during this stage of infectious diseaseIf the patient does not recover, the disease may be fatal

Stages of Infectious Disease:ConvalescenceThis is a period of recoveryDamaged tissues and other body systems are repaired

Factors affecting the length of the convalescence period:Amount of damageNature of the pathogenSite of infectionOverall health of the patientDepending of the disease, patients may be infectious during any stage of disease including the convalescence period

Portal of Exits:Pathogens must leave the body to infect other hostsMany portals of exit are identical to portals of entry

Pathogens often leave the body in:SecretionsEarwax, tears, nasal secretions, saliva, sputum, respiratory droplets, blood, genital secretions, and milkExcretionsFeces and urine

Chain of Infection: Reservoir of InfectionMost pathogens cannot survive for long outside of their hostPathogens encounter a wide variety of stresses in the external environmentSites were pathogens persist and go on to cause other infections are termed reservoirs of infection

There are three major reservoirs of human infection:Animal reservoirsHuman reservoirsNon-living reservoirs

Animal reservoirsMany pathogens that infect domestic or wild animals can also infect humansDiseases that spread from animals to humans are called zoonoses or zoonotic diseasesThere are currently over 150 zoonotic disease knownThe animals involved serve as a reservoir of the pathogenPathogens can be spread from animal to human through direct contact with animals and their wastes, by eating meat, and by bloodsucking arthropodsHumans are often a "dead-end" host for zoonotic pathogensHowever, diseases transmitted by arthropod vectors can travel from humans to animals

zoonoses or zoonotic diseasesDiseases that spread from animals to humans are called

Human reservoirsOther people with active infections are reservoirs of infection if the disease is a communicable diseaseCarriers can remain asymptomatic and infective for years(e.g., syphilis and AIDS)Some carriers go through the stages of disease while others may never develop illness at allThese healthy carriers presumably have immune systems that protect them from becoming sick(e.g., typhoid Mary)

Non-living reservoirs

Soil, water, and food can all harbor pathogenic microbesThe bacteria that cause tetanus and botulism live in soilWater can contain many types of pathogens especially if contaminated by fecesMeats, vegetables, and milk can all contain microbes able to survive in such an environmentPasteurization is a method that uses heat to reduce the number of microorganisms present in liquids such as milk