Infectious Disease Process

Unit 2: Infectious Disease Process Objectives

• The unit defines related terms in infectious disease
• Dynamics of disease transmission are discussed
• Mechanisms of disease transmission are classified

Pathogen

• A pathogen is an organism that causes disease in its host
• Three major types are bacteria, viruses and fungi
• Other disease-causing groups include protozoa and helminths

Bacteria

• Bacterial cells lack a true nucleus and membrane-bound organelles
• Most bacteria reproduce through binary fission
• During binary fission, the cell grows to twice its starting size and splits in two
• Each offspring receives a complete copy of the essential genetics

Bacterial Growth Graph Phases

• Lag phase: no increase in the number of living bacterial cells
• Log phase: exponential increase in the number of living bacterial cells
• Stationary phase: plateau in number of living bacterial cells with the rate of cell division and death roughly equal
• Death or decline phase: exponential decrease in the number of living bacterial cells

Viruses

• Viruses lack a cellular structure and can only reproduce inside other living cells
• Viruses attach to a host cell, inject DNA, and use the host cell to reproduce

Fungi

• Most fungi are multicellular; yeasts are unicellular
• Fungi produce spores that disperse from the parent organism
• Fungi may reproduce sexually or asexually

Reservoirs

• A reservoir is a habitat where an agent normally lives, grows, and multiplies
• Reservoirs include humans, animals, and the environment
• The reservoir may or may not be the source from which an agent transfers to a host
• There are human, animal and environmental reservoirs

Human Reservoirs

• Many common infectious diseases are spread from person to person without intermediaries through human reservoirs such as STDs, measles, and mumps
• Human reservoirs may or may not show the effects of illness (symptomatic vs. asymptomatic)
• A carrier is an asymptomatic person who can transmit the pathogen
• Incubatory carriers can transmit the agent during the incubation period before clinical illness begins
• Convalescent carriers have recovered from their illness but can still transmit the pathogen
• Chronic carriers harbor a pathogen like the hepatitis B virus or Salmonella Typhi for months or years after initial infection

Animal Reservoirs

• Many diseases transmit from animal to animal, with humans as incidental hosts
• Zoonosis is an infectious disease transmissible from vertebrate animals to humans under natural conditions
• Recognized zoonotic diseases include anthrax (sheep), plague (rodents), trichinellosis/trichinosis (swine), and rabies (bats, raccoons, dogs, and other mammals)
• Newly recognized infectious diseases in humans, including HIV/AIDS, Ebola infection and SARS, are thought to have emerged from animal hosts

Environmental Reservoirs

• Plants, soil, and water are reservoirs for some infectious agents
• Many fungal agents live and multiply in the soil
• Outbreaks of Legionnaires' disease trace to water supplies in cooling towers and evaporative condensers of the causative organism Legionella pneumophila

Types of Infections

• Localized infection is confined to one area of the body or organ system, examples including an ear infection or a localized cancer
• Systemic infection affects the entire body, rather than a single organ or body part, with examples including influenza
• Diabetes mellitus is an example of a systemic disease, though not caused by infection

Periods of Disease

• Transmission can occur during any of the periods of disease, depending upon the pathogen, the disease and the individual infected
• Five periods of disease (a.k.a. stages or phases) include Incubation, Prodromal, Illness, Decline, and Convalescence periods

Incubation Period

• Occurs in an acute disease after the initial entry of the pathogen into the host
• Pathogen begins multiplying in the host
• Insufficient pathogen particles are present to cause signs and symptoms of disease
• The incubation period can last days in acute diseases to months or years in chronic diseases
• Length of the incubation period depends on the strength of the pathogen, host immune defenses, site of infection, type of infection, and the size infectious dose received

Periods of Disease

• The Prodromal period occurs after the incubation period, with the pathogen continuing to multiply and the host experiencing general signs and symptoms of illness
• The Period of Illness, follows the prodromal period, during which the signs and symptoms of disease are most obvious and severe
• The Period of Decline follows the period of illness, with the number of pathogen particles beginning to decrease and signs and symptoms of illness beginning to decline
• Patients may be susceptible to secondary infections during the decline period because of weakened immune systems
• The Period of Convalescence is the final period, with the patient generally returning to normal functions although some diseases may inflict permanent damage

Transmission

• Transmission refers to the means by which an infectious agent may be transmitted from its natural reservoir to a susceptible host
• Direct transmission includes direct contact and droplet spread

Direct Contact Transmission

• Physical interaction results in transmission of the agent, such as mononucleosis
• It includes contact with soil or vegetation harboring infectious organisms eg. Hookworm via direct contact with contaminated soil
• Personal contact is common type of direct contact referring to the physical and usually "skin to skin" transmission of the pathogen
• Sexual contact is a form of direct contact sexual contact, such as HIV and Chlamydia (caused by the bacteria Chlamydia trachomatis)
• Mother to baby transmission refers to the delivery of infectious agents from a woman to her child during pregnancy, labor, delivery or breastfeeding, eg. HIV/AIDS

Droplet Spread Transmission

• Refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking
• A direct transmission because is classified transmission by direct spray over a few feet, before the droplets fall to the ground
• Pertussis is an example of disease transmitted from an infectious patient to a susceptible host by droplet spread

Indirect Transmission

• Refers to the transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects (vehicles), or animate intermediaries (vectors)
• Two major means of indirect transmission include airborne transmission and vector borne
• Airborne transmission occurs when infectious agents are carried by dust or droplet nuclei (dried residue of less than 5 microns in size) suspended in air
• Droplet nuclei may remain suspended in the air for long periods and may be blown over great distances, unlike droplets, eg. measles

Indirect Transmission Vehicles and Vectors

• Vehicles that may indirectly transmit an infectious agent include food, water, biologic products (blood), and fomites
• A vehicle may passively carry a pathogen, like hepatitis A virus
• A vehicle may provide an environment in which the agent grows, multiplies, or produces toxin, like improperly canned foods provide an anaerobic environment that supports production of botulinum toxin by Clostridium botulinum
• Vectors like mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent
• Examples of mechanical transmission are flies carrying Shigellaon their appendages and fleas carrying Yersinia pestis in their gut
• In biologic transmission, the causative agent of malaria or guinea worm disease undergoes maturation in an intermediate host before transmission to humans

Chain of Infection

• Describes how transmission occurs when an agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host

Prevention of Disease

• Primary prevention aims to prevent disease or injury before it ever occurs, usually by removing an element from the chain of infection
• Secondary prevention aims to reduce the impact of a disease or injury that has already occurred, usually by early detection and treatment
• Tertiary prevention aims to soften the impact of an ongoing illness or injury that has lasting effects

Primary Prevention Examples

• Aims to prevent disease or injury before it ever occurs
• Done by preventing exposures to hazards, altering unhealthy or unsafe behaviors, and increasing resistance to disease or injury
• Includes legislation and enforcement to ban or control hazardous products or to mandate safe and healthy practices
• Includes education about healthy and safe habits
• Includes immunization against infectious diseases

Secondary Prevention Examples

• Aims to reduce the impact of a disease or injury that has already occurred
• Done by detecting and treating disease or injury as soon as possible, encouraging personal strategies to prevent reinjury or recurrence, and implementing programs to return people to their original health and function'
• Includes regular exams and screening tests to detect disease in its earliest stages
• Includes daily, low-dose aspirins and/or diet and exercise programs to prevent further heart attacks or strokes
• Includes suitably modified work so injured or ill workers can return safely to their jobs

Tertiary Prevention Examples

• Aims to soften the impact of an ongoing illness or injury
• Done by helping people manage long-term, often-complex health problems and injuries to improve their ability to function, their quality of life and their life expectancy
• Includes cardiac or stroke rehabilitation programs and chronic disease management programs
• Includes support groups that allow members to share strategies for living well
• Includes vocational rehabilitation programs to retrain workers for new jobs when they have recovered as much as possible